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/wrap_GraphicsReadback.cpp

@@ -0,0 +1,95 @@
+/**
+ * Copyright (c) 2006-2021 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.
+ **/
+
+// LOVE
+#include "wrap_GraphicsReadback.h"
+#include "data/ByteData.h"
+#include "image/ImageData.h"
+
+namespace love
+{
+namespace graphics
+{
+
+GraphicsReadback *luax_checkgraphicsreadback(lua_State *L, int idx)
+{
+	return luax_checktype<GraphicsReadback>(L, idx);
+}
+
+int w_GraphicsReadback_isComplete(lua_State *L)
+{
+	GraphicsReadback *t = luax_checkgraphicsreadback(L, 1);
+	luax_pushboolean(L, t->isComplete());
+	return 1;
+}
+
+int w_GraphicsReadback_hasError(lua_State *L)
+{
+	GraphicsReadback *t = luax_checkgraphicsreadback(L, 1);
+	luax_pushboolean(L, t->hasError());
+	return 1;
+}
+
+int w_GraphicsReadback_wait(lua_State *L)
+{
+	GraphicsReadback *t = luax_checkgraphicsreadback(L, 1);
+	t->wait();
+	return 0;
+}
+
+int w_GraphicsReadback_update(lua_State *L)
+{
+	GraphicsReadback *t = luax_checkgraphicsreadback(L, 1);
+	luax_catchexcept(L, [&]() { t->update(); });
+	return 0;
+}
+
+int w_GraphicsReadback_getBufferData(lua_State *L)
+{
+	GraphicsReadback *t = luax_checkgraphicsreadback(L, 1);
+	luax_pushtype(L, t->getBufferData());
+	return 1;
+}
+
+int w_GraphicsReadback_getImageData(lua_State *L)
+{
+	GraphicsReadback *t = luax_checkgraphicsreadback(L, 1);
+	luax_pushtype(L, t->getImageData());
+	return 1;
+}
+
+static const luaL_Reg w_GraphicsReadback_functions[] =
+{
+	{ "isComplete", w_GraphicsReadback_isComplete },
+	{ "hasError", w_GraphicsReadback_hasError },
+	{ "wait", w_GraphicsReadback_wait },
+	{ "update", w_GraphicsReadback_update },
+	{ "getBufferData", w_GraphicsReadback_getBufferData },
+	{ "getImageData", w_GraphicsReadback_getImageData },
+	{ 0, 0 }
+};
+
+extern "C" int luaopen_graphicsreadback(lua_State *L)
+{
+	return luax_register_type(L, &GraphicsReadback::type, w_GraphicsReadback_functions, nullptr);
+}
+
+} // graphics
+} // love