Make cogl_texture_draw_and_read_to_bitmap public

This makes the internal api  _cogl_texture_draw_and_read public as
cogl_texture_draw_and_read_to_bitmap() so that applications can manually
use this api to handle fallbacks when it's not possible to directly read
a textures contents.

The intention is to no longer automatically render and read-back
textures as a fallback on drivers that don't support glGetTexImage2D
since this fallback depends on the idea of a current framebuffer which
we'd like to get rid of and there is no feedback that the contents of
the framebuffer have been destroyed if the fallback is hit.

Reviewed-by: Neil Roberts <neil@linux.intel.com>