Check and handle GL ES gracefully.

[matwey]

Many ARM based hardware has only GL ES (and no GL) support, but they are still provide conventional Linux based work environment.

• OpenGLES doesn't support 1D textures, that are required for colormaps.
• OpenGLES doesn't support UNPACK_SWAP_BYTES
• OpenGLES doesn't support normalized 16-bit texture types.
  • https://www.khronos.org/registry/OpenGL/extensions/EXT/EXT_texture_norm16.txt
  • normalized texture types are mostly useless since the bytes cannot be swapped during unpack

References:

• https://www.khronos.org/registry/OpenGL/specs/es/2.0/es_cm_spec_2.0.pdf