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/metal/Texture.h

@@ -57,7 +57,6 @@ private:
 
 	void uploadByteData(PixelFormat pixelformat, const void *data, size_t size, int level, int slice, const Rect &r) override;
 	void generateMipmapsInternal() override;
-	void readbackImageData(love::image::ImageData *imagedata, int slice, int mipmap, const Rect &rect) override;
 
 	id<MTLTexture> texture;
 	id<MTLTexture> msaaTexture;