Refactor CC3Scene drawing and add framebuffer GL access.

Refactor CC3Scene drawScene method. Add drawSceneContent method for customizing
app scene drawing. Add framebuffer methods to CC3OpenGL cluster. Viewport tracked
per camera. Change CC3Scene iVar naming. Improve compilation warning resilience,
including shadow variables and pointer-interger conversion. Add additional
compilation warnings to CC3DemoMashUp project. Fix issue in resetting bound
texture in 2D cache. Fix issue with CC3GLSLVariable size tracking.
Add framebuffer-based CC3RenderSurfaces.

Add CC3RenderSurface protocol & CC3FramebufferRenderSurface implementation.
Add basic render to texture in CC3DemoMashUp. Create CC3Texture, CC3GLTexture2D &
CC3GLTextureCube from CGImages already in memory. Create empty CC3GLTexture2D &
CC3GLTextureCube for use as a rendering surface. CC3PFXResource internally cache
CC3GLTexture instead of CC3Texture. Add GL get int, float & string methods.
Refactor CC3GLView to use framebuffer-based CC3RenderSurfaces.

Refactor OGLES2 CC3GLView to use framebuffer-based CC3RenderSurfaces managed by
a CC3GLViewSurfaceManager instance. CC3UIViewController now requires its view be
a CC3GLView. OGLES1 & OGLOSX pending. Fix CC3Camera frustum not built when
accessed for shadows. Shadow volumes now work when multisampling is active.
Add ability to easily draw in clip-space for backdrops and post-processing.

Add CC3ClipSpaceNode mesh node to easily draw rectangles in clip-space.
Add backdrop property to CC3Scene. CC3Layer subclassed from CCLayer and no longer
draws the 3D scene backdrop. Shadows painted with a CC3ClipSpaceNode subclass.
Add backdrop to CC3DemoMashUp scenes. CC3Scene depth clearing now controlled via
drawSceneContentsWithVisitor: method. Deprecated shouldClearDepthBuffer.
Add post-rendering image processing.

Refactor CC3RenderSurface protocols and classes. Add ability to resize textures
attached to framebuffers. Add grayscale image processing option to CC3DemoMashUp.
Add TV model to CC3DemoMashUp to display render-to-texture scene.