mesa/glsl: add bitmask to track stages a program was linked against

This will be used to enable us to store the current gl_program
rather than gl_shader_program in the gl_pipline_object allowing
us to simplify handing of validation.

Also we should not be depending on _LinkedShader for this information
as it may contain shaders from a failed linking attempt rather than
the current program still in use.

We could also use this mask to iterate over the stages during linking
with _mesa_bit_scan() rather then the current method of NULL checking
each stage.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

src/compiler/glsl/linker.cpp

@@ -4889,6 +4889,7 @@ link_shaders(struct gl_context *ctx, struct gl_shader_program *prog)
          }
 
          prog->_LinkedShaders[stage] = sh;
+         prog->data->linked_stages |= 1 << stage;
       }
    }
 

src/mesa/main/mtypes.h

@@ -2652,6 +2652,9 @@ struct gl_shader_program_data
    GLboolean LinkStatus;   /**< GL_LINK_STATUS */
    GLboolean Validated;
    GLchar *InfoLog;
+
+   /* Mask of stages this program was linked against */
+   unsigned linked_stages;
 };
 
 /**