Add sync and async texture and buffer readback APIs.

- Add imagedata = love.graphics.readbackTexture(texture, [slice, mipmap, x, y, w, h, [dest, destx, desty]]).
- Add readback = love.graphics.readbackTextureAsync(texture, [slice, mipmap, x, y, w, h, [dest, destx, desty]]).
- Add bytedata = love.graphics.readbackBuffer(buffer, [offset, size, [dest, destoffset]]).
- Add readback = ove.graphics.readbackBufferAsync(buffer, [offset, size, [dest, destoffset]]).
- Deprecate Texture:newImageData.

The async variants return a new GraphicsReadback object. It has the following methods:
- isComplete()
- hasError()
- wait()
- getBufferData()
- getImageData()
- update() (called automatically every frame by love, not normally needed).

Support notes:
- readbackBuffer and readbackBufferAsync require buffer copying support.
- readbackTexture with a render target (canvas) is always supported. readbackTexture with a non-render-target requires copy-texture-to-buffer support.
- readbackTextureAsync with a render target requires copy-render-target-to-buffer support. readbackTextureAsync with a non-render-target requires copy-texture-to-buffer support.

src/modules/graphics/opengl/GraphicsReadback.cpp

@@ -0,0 +1,126 @@
+/**
+ * Copyright (c) 2006-2022 LOVE Development Team
+ *
+ * This software is provided 'as-is', without any express or implied
+ * warranty.  In no event will the authors be held liable for any damages
+ * arising from the use of this software.
+ *
+ * Permission is granted to anyone to use this software for any purpose,
+ * including commercial applications, and to alter it and redistribute it
+ * freely, subject to the following restrictions:
+ *
+ * 1. The origin of this software must not be misrepresented; you must not
+ *    claim that you wrote the original software. If you use this software
+ *    in a product, an acknowledgment in the product documentation would be
+ *    appreciated but is not required.
+ * 2. Altered source versions must be plainly marked as such, and must not be
+ *    misrepresented as being the original software.
+ * 3. This notice may not be removed or altered from any source distribution.
+ **/
+
+#include "GraphicsReadback.h"
+#include "Buffer.h"
+#include "Texture.h"
+#include "graphics/Graphics.h"
+#include "data/ByteData.h"
+
+namespace love
+{
+namespace graphics
+{
+namespace opengl
+{
+
+GraphicsReadback::GraphicsReadback(love::graphics::Graphics *gfx, ReadbackMethod method, love::graphics::Buffer *buffer, size_t offset, size_t size, data::ByteData *dest, size_t destoffset)
+	: love::graphics::GraphicsReadback(gfx, method, buffer, offset, size, dest, destoffset)
+{
+	// Immediate readback of readback-type buffers doesn't need a staging buffer.
+	if (method != READBACK_IMMEDIATE || buffer->getDataUsage() != BUFFERDATAUSAGE_READBACK)
+	{
+		stagingBuffer = gfx->getTemporaryBuffer(size, DATAFORMAT_FLOAT, 0, BUFFERDATAUSAGE_READBACK);
+		gfx->copyBuffer(buffer, stagingBuffer, offset, 0, size);
+	}
+
+	if (method == READBACK_IMMEDIATE)
+	{
+		if (stagingBuffer.get())
+		{
+			status = readbackBuffer(stagingBuffer, 0, size);
+			gfx->releaseTemporaryBuffer(stagingBuffer);
+		}
+		else
+		{
+			status = readbackBuffer(buffer, offset, size);
+		}
+	}
+	else
+	{
+		sync.fence();
+	}
+}
+
+GraphicsReadback::GraphicsReadback(love::graphics::Graphics *gfx, ReadbackMethod method, love::graphics::Texture *texture, int slice, int mipmap, const Rect &rect, image::ImageData *dest, int destx, int desty)
+	: love::graphics::GraphicsReadback(gfx, method, texture, slice, mipmap, rect, dest, destx, desty)
+{
+	size_t size = getPixelFormatSliceSize(textureFormat, rect.w, rect.h);
+
+	if (method == READBACK_IMMEDIATE)
+	{
+		void *dest = prepareReadbackDest(size);
+
+		love::thread::Lock lock(imageData->getMutex());
+
+		// Direct readback without copying avoids the need for a staging buffer,
+		// and lowers the system requirements of immediate RT readback.
+		Texture *t = (Texture *) texture;
+		t->readbackInternal(slice, mipmap, rect, imageData->getWidth(), size, dest);
+
+		status = STATUS_COMPLETE;
+	}
+	else
+	{
+		stagingBuffer = gfx->getTemporaryBuffer(size, DATAFORMAT_FLOAT, 0, BUFFERDATAUSAGE_READBACK);
+
+		gfx->copyTextureToBuffer(texture, stagingBuffer, slice, mipmap, rect, 0, 0);
+		sync.fence();
+	}
+}
+
+GraphicsReadback::~GraphicsReadback()
+{
+}
+
+void GraphicsReadback::wait()
+{
+	if (status != STATUS_WAITING)
+		return;
+
+	sync.cpuWait();
+	update();
+}
+
+void GraphicsReadback::update()
+{
+	if (status != STATUS_WAITING)
+		return;
+
+	if (sync.isComplete())
+	{
+		if (stagingBuffer.get())
+			status = readbackBuffer(stagingBuffer, 0, stagingBuffer->getSize());
+		else
+			status = STATUS_ERROR;
+
+		if (stagingBuffer.get())
+		{
+			auto gfx = Module::getInstance<love::graphics::Graphics>(Module::M_GRAPHICS);
+			if (gfx != nullptr)
+				gfx->releaseTemporaryBuffer(stagingBuffer);
+			stagingBuffer.set(nullptr);
+		}
+	}
+}
+
+} // opengl
+} // graphics
+} // love