gl: bind position attribute to location 0

Full GL treats the position attribute specially and requires that it
must be bound to generic attribute location 0 unlike GLES 2.0 or
GL 3.2 core. We now make sure to unconditionally bind the
cogl_position_in attribute to location 0 before linking any glsl program
in cogl.

For reference the relevant part of the GL 3.0 spec that covers these
semantics is Section 2.7 "Vertex Specification" pg 27

After this change there was one remaining problem in
test-custom-attributes where the test_short_verts() test was using its
own "pos" attribute instead of cogl_position_in and so cogl wasn't able
to ensure it would be bound to location 0.

This updates the test to use cogl_position_in but to work around the
fact that glVertexPointer doesn't support UNSIGNED_SHORT components we
force the test to use the glsl backend by setting a shader snippet on
the pipeline.

https://bugs.freedesktop.org/show_bug.cgi?id=67548

Reviewed-by: Neil Roberts <neil@linux.intel.com>