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/image/magpie/ddsHandler.h

@@ -48,7 +48,7 @@ public:
 	bool canParseCompressed(Data *data) override;
 	StrongRef<ByteData> parseCompressed(Data *filedata,
 	        std::vector<StrongRef<CompressedSlice>> &images,
-	        PixelFormat &format, bool &sRGB) override;
+	        PixelFormat &format) override;
 
 }; // DDSHandler