ImageData can track whether they're meant to be loaded into a Texture with a non-sRGB format when gamma correct rendering is enabled.

Fixes #1556.

* Add ImageData/CompressedImageData:setLinear and ImageData/CompressedImageData:isLinear.
  The flag is used as a hint when loading a texture from the data to determine if the format should not be treated as sRGB-encoded. The 'linear' flag in newImage overrides this.

* love.graphics.readbackTexture automatically sets the linear flag on ImageData it returns when the texture's format is linear.
  This allows an ImageData generated via readback to be fed back into a new Texture and the format will match the original Canvas.

* Also clean up some image decoding code.

src/modules/graphics/GraphicsReadback.h

@@ -103,6 +103,7 @@ protected:
 	StrongRef<love::image::ImageData> imageData;
 	Rect rect = {};
 	PixelFormat textureFormat = PIXELFORMAT_UNKNOWN;
+	bool isFormatLinear = false;
 	int imageDataX = 0;
 	int imageDataY = 0;