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/Texture.h

@@ -245,8 +245,6 @@ public:
 
 	void generateMipmaps();
 
-	love::image::ImageData *newImageData(love::image::Image *module, int slice, int mipmap, const Rect &rect);
-
 	virtual void copyFromBuffer(Buffer *source, size_t sourceoffset, int sourcewidth, size_t size, int slice, int mipmap, const Rect &rect) = 0;
 	virtual void copyToBuffer(Buffer *dest, int slice, int mipmap, const Rect &rect, size_t destoffset, int destwidth, size_t size) = 0;
 
@@ -313,7 +311,6 @@ protected:
 
 	bool supportsGenerateMipmaps(const char *&outReason) const;
 	virtual void generateMipmapsInternal() = 0;
-	virtual void readbackImageData(love::image::ImageData *imagedata, int slice, int mipmap, const Rect &rect) = 0;
 
 	bool validateDimensions(bool throwException) const;