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/FenceSync.cpp

@@ -45,6 +45,22 @@ bool FenceSync::fence()
 	return !wasActive;
 }
 
+bool FenceSync::isComplete() const
+{
+	if (sync == 0)
+		return true;
+
+	GLenum status = glClientWaitSync(sync, 0, 0);
+
+	if (status == GL_ALREADY_SIGNALED || status == GL_CONDITION_SATISFIED)
+		return true;
+
+	if (status == GL_WAIT_FAILED)
+		return true;
+
+	return false;
+}
+
 bool FenceSync::cpuWait()
 {
 	if (sync == 0)