Added support for Texture Cubes

DDS and PVR loading supports texture cubes.
FrameBuffer::createScreenshot lets you choose the format (RGB or RGBA)
Added Texture::setData so that it is possible to set change texture data after the texture has been created.

Filtered file selection of #1513 and #1528
I think I tried this before to get rid of the warning. However it was not resolving properly on one of the platforms. 
Have you tried this on all platforms ? I can't remember which platform though.