Add profiling codes to measure time to invoke DrawTextureCubeEx

Profiling result shows that vertex buffer objects (VBOs) with high vertex
count (80,000 vertices or so) make the rendering really slow in Radeon HD
6870, while it doesn't make much difference on GeForce GT 650M.

The reason is speculated to be related with memory cache, but I don't have
explicit explanation.
Make intermediate LOD use dedicated VBO to enhance speed of DrawTextureCubeEx