[cogl journal] If we are only flushing one quad use a TRIANGLE_FAN

[cogl journal] If we are only flushing one quad use a TRIANGLE_FAN

At the moment Cogl doesn't do much batching of quads so most of the time we
are flushing a single quad at a time.  This patch simplifies how we submit
those quads to OpenGL by using glDrawArrays with GL_TRIANGLE_FAN mode
instead of sending indexed vertices using GL_TRIANGLES mode.

Note: I hope to follow up soon with changes that improve our batching and
also move the indices into a VBO so they don't need to be re-validated every
time we call glDrawElements.