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.
Sasha Szpakowski
8c13943f64