Add CC3GLTexture class cluster to represent loaded textures.

Add CC3GLTexture class cluster to represent loaded textures. Replace instance
of CC3Texture2D in CC3Texture with CC3GLTexture instance. CC3GLTexture instances
are cached to avoid duplicate loading. Add texture methods to CC3OpenGL class
cluster.
Fixes to CC3PODLight.

Interpret GL position of directional POD lights the same way as PVRShaman.
Extract spot exponent from POD file lights.
Texturing using targets.

Add texturing to targets other than GL_TEXTURE_2D.
Add cube-map texture loading to CC3Texture & CC3GLTextureCube.

Add cube-map texture loading to CC3Texture & CC3GLTextureCube. Add ability to mix
standard GLSL variable names with effect-defined variable names in shaders loaded
via a PFX effects file. Add cube-map texture to reflective teapot and skybox in
CC3DemoMashUp. Fix iOS 4.0 runtime issue in CC3UIViewController viewDidLayoutSubviews.
Merge pull request #9 from billhollings/master

Cube mapping