Enhance OpenGL RTV and DSV with comprehensive framebuffer support
OpenGLRenderTargetView:
- Add RenderTargetType enum for different texture types
- Add RenderTargetViewDesc struct with mip level, array slice, layer info
- Add Initialize() with desc parameter for 2D/2DArray/3D/Cube
- Add InitializeCubemap() for cubemap faces
- Add Bind(count) overload for multiple framebuffers
- Add Clear() methods for color and depth-stencil
- Add static BindFramebuffer/UnbindFramebuffer methods
OpenGLDepthStencilView:
- Add DepthStencilType enum for different texture types
- Add DepthStencilViewDesc struct with mip level, array slice, layer info
- Add Initialize() with desc parameter for 2D/2DArray/Cube
- Add InitializeCubemap() for cubemap faces
- Add ClearDepth, ClearStencil, ClearDepthStencil methods
- Add static BindFramebuffer/UnbindFramebuffer methods