Fix an i965 assertion failure on glClear()

Fix an i965 assertion failure on glClear()

While running conform with render-to-texture:

	conform -d 33 -v 2 -t -direct

the i965 driver failed this assertion:

intel_clear.c:77: intel_clear_tris: Assertion `(mask & ~((1 << BUFFER_BACK_LEFT) | (1 << BUFFER_FRONT_LEFT) | (1 << BUFFER_DEPTH) | (1 << BUFFER_STENCIL))) == 0' failed.

The problem is that intel_clear_tris() is called by intelClear() to
clear any and all of the available color buffers, but intel_clear_tris()
actually only handles the back left and front left color buffers; so
the assertion fails as soon as you try to clear a non-standard color
buffer.

The fix is to have intelClear() only call intel_clear_tris() with
buffers that intel_clear_tris() can support.  intelClear() already backs
down to _swrast_Clear() for all buffers that aren't handled explicitly.