If the loaded shader has uniforms, they will be exposed to the UI, so the users will be able to modify the shader parameters easily. Currently only works with float uniforms.
This new feature allows users to load a .shader file (must be a GLES2 Godot shader) that will modify the image itself. This feature is experimental and possibly buggy, not all shaders are working properly and I'm not sure yet why. As such, this feature may not be included in v0.8 stable.
Had to move some palette png code around in order to make it possible to use these image files as palettes. Hopefully loading .pxo files should be next. Addresses #280
