mesa: Respect GL_RASTERIZER_DISCARD for various meta-type operations.

mesa: Respect GL_RASTERIZER_DISCARD for various meta-type operations.

From the EXT_transform_feedback spec:

    Primitives can be optionally discarded before rasterization by calling
    Enable and Disable with RASTERIZER_DISCARD_EXT. When enabled, primitives
    are discared right before the rasterization stage, but after the optional
    transform feedback stage. When disabled, primitives are passed through to
    the rasterization stage to be processed normally. RASTERIZER_DISCARD_EXT
    applies to the DrawPixels, CopyPixels, Bitmap, Clear and Accum commands as
    well.

And the GL 3.2 spec says it applies to ClearBuffer* as well.

Reviewed-by: Brian Paul <brianp@vmware.com>