Fix MRT and clean-up cubemap FBOs

[GlennFolker]

• Made CubemapSide.all = values().
• Completely remove FrameBufferCubemap's nextSide() as it completely chokes when another FBO is bound while it is already bound, causing currentSide to go back to -1 when it is rebound, resulting in an eternal while-loop.
• Refactored:
  • disposeColorTexture() ->disposeTexture()
  • attachFrameBufferColorTexture() -> attachTexture() with an attachment parameter, e.g. Gl.colorAttachment0 and Gl.depthAttachment