Added document section on UI textures.
Added more accurate but more expensive raycast for BillboardSet, which approximates the individual billboards as spheres and gives the billboard index as the subobject. RAY_TRIANGLE level raycast must be used to enable it. Removed the RAY_AABB_NOSUBOBJECTS raycast level, as it was not used anywhere.
Added Image pixel manipulation, resize and clear functions. Added Image profiling blocks. Closes #171.
Merge remote-tracking branch 'upstream/master'