Decode images using a 32 bits RGB format instead of 24bits.

[nical]

The problem with 24 bits rgb is that it is not very well supported. OpenGL exposes it but looks like only intel handles it somewhat properly and D3D does not expose it which is an issue if servo ends up running WebRender over ANGLE on windows (I don't know if that's the plan or not, but it is the current plan for gecko).

cc @glennw

[nox]

@nical Is this still an issue? Where should that be done?

[nical]

I don't know if this is still an issue in servo. If it is, it should be addressed where the images are decoded I suppose, or somewhere else before the image is passed to WebRender.