mesa: retain gl_shader_programs after glDeleteProgram if they are in use

mesa: retain gl_shader_programs after glDeleteProgram if they are in use

Fixes regressions from c505d6d852220f4aaaee161465dd2c579647e672.

Switching from using gl_shader_program to gl_program for the pipline
objects CurrentProgram array meant we were freeing gl_shader_programs
immediately after glDeleteProgram was called, but the spec states
the program should only get deleted once it is no longer in use.

To work around this we add a new ReferencedPrograms array to track
gl_shader_programs in use.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>