Added CompressedTexImage2D and constants

Added function:

glCompressedTexImage2D

and constants:
GL_COMPRESSED_RGBA_S3TC_DXT1_EXT
GL_COMPRESSED_RGBA_S3TC_DXT3_EXT
GL_COMPRESSED_RGBA_S3TC_DXT5_EXT

From issue
https://github.com/go-gl/gl/issues/103