Use cogl_read_pixels in the cogl texture draw-and-read fallback

Use cogl_read_pixels in the cogl texture draw-and-read fallback

The fallback for when glGetTexImage is not available draws parts of
the texture to the framebuffer and uses glReadPixels to extract the
data. However it was using cogl_rectangle to draw and then immediately
using raw glReadPixels to fetch the data. This won't cause a journal
flush so the rectangle won't necessarily have hit the framebuffer
yet. Instead it now uses cogl_read_pixels which does flush the
journal.