check the glsl version during init

This adds a check for the glsl version during driver init which gets
stored in ctx->glsl_major and ctx->glsl_minor.

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

(cherry picked from commit 9bde48bda6d602dd536c3536d56d2ff7545802c3)

cogl/cogl-context-private.h

@@ -75,6 +75,9 @@ struct _CoglContext
   const CoglDriverVtable *driver_vtable;
   const CoglTextureDriver *texture_driver;
 
+  int glsl_major;
+  int glsl_minor;
+
   /* Features cache */
   unsigned long features[COGL_FLAGS_N_LONGS_FOR_SIZE (_COGL_N_FEATURE_IDS)];
   CoglFeatureFlags feature_flags; /* legacy/deprecated feature flags */