i965: Reduce a single GL_QUADS to GL_TRIANGLE_FAN.

i965: Reduce a single GL_QUADS to GL_TRIANGLE_FAN.

This is similar to the GL_QUAD_STRIP -> TRIANGLE_STRIP optimization --
the GS usage to split the quads into tris is a huge bottleneck, so a
quick check improves glean blendFunc time massively (width * height of
the window of single-pixel GL_QUADS, many many times).  This may also
end up helping with cairo performance, which sometimes ends up drawing
a single quad.