cogl-material: Always reset the GLSL program to zero when flushing

cogl-material: Always reset the GLSL program to zero when flushing

When flushing a fixed-function or arbfp material it would always call
disable_glsl to try to get rid of the previous GLSL shader. This is
needed even if current_use_program_type is not GLSL because if an
application calls cogl_program_uniform then Cogl will have to bind the
program to set the uniform. If this happens then it won't update
current_use_program_type presumably because the enabled state of arbfp
is still valid.

The problem was that disable_glsl would only select program zero when
the current_use_program_type is set to GLSL which wouldn't be the case
if cogl_program_uniform was called. This patch changes it to just
directly call _cogl_gl_use_program_wrapper(0) instead of having a
separate disable_glsl function. The current program is cached in the
cogl context anyway so it shouldn't cause any extra unnecessary GL
calls.

http://bugzilla.clutter-project.org/show_bug.cgi?id=2232