cogl-pipeline-glsl: Fix reusing shaders for shared ancestors

cogl-pipeline-glsl: Fix reusing shaders for shared ancestors

The check for whether we can reuse a program we've already generated
was only being done if the pipeline already had a
glsl_program_state. When there is no glsl_program_state it then looks
for the nearest ancestor it can share the program with. It then
wasn't checking whether that ancestor already had a GL program so it
would start generating the source again. It wouldn't however compile
that source again because _cogl_pipeline_backend_glsl_end does check
whether there is already a program. This patch moves the check until
after it has found the glsl_program_state, whether or not it was found
from an ancestor or as its own state.