FX 8 3D: Change Mesh's indexBuffer type from unsigned int to unsigned short

While doing the Raspberry Pi porting work, we realized that Pi doesn't support uint indices and OpenGL ES 2 doesn't require it:

"Indexing support with ubyte and ushort indices is supported. Support for uint indices is not required
by OpenGL ES 2.0."

We will have to specify that indices, points and texcoords be limited to 64K. We will need to provide method to query max. vertices in future release when we add uint for platform that support it.