Fix gl version using strings.

This was causing 'is_legacy' and 'is_core' to fail
as they expected ints.
Fix syntax error in TextureCoordAttribute.

Add missing self keyword.
Fix rendering textured quad in legacy.

Don't use location=.
Remove generic attribute since we're using FF.
Set gl_TexCoord[0] to gl_MultiTexCoord0.
Bind the sampler to texture unit 0.
Remove other location= calls for legacy.
Trivial change to comment for consistency.
Add gl_enum_dtype and error check in dtype_gl_enum.

gl_enum_dtype does the opposite of dtype_gl_enum.
Remove ability to pass handle to new buffer.

This wasn't doing what it stated (preserving existing
buffer data / size).
Re-add it later if / when needed.
Re-write texture module.

Remove pil_texture, rolled into texture.py.
Make texture more like vertex_buffer.
Update texture example.