Merge commit 'cogl-reorg-draw-buffers'

* cogl-reorg-draw-buffers: (38 commits)
  [test-fbo] greatly simplify the test
  [tests] test-backface-culling: test culling with offscreen rendering
  [tests] Adds test-cogl-readpixels.c for very basic cogl_read_pixels testing
  [tests] Adds test-cogl-offscreen to validate offscreen draw buffer
  [tests] test-cogl-viewport tests semantics of over/under size viewports
  [test-texture-fbo] comment the colors defined in corner_colors
  Add a conformance test for clutter_texture_new_from_actor
  [cogl-texture-2d-sliced] allow COGL_FORMAT_ANY with _new_with_size()
  [texture] fix rounding when calculating update_fbo viewport offset
  [texture] switch to a new design for handling offscreen rendering
  [texture] split out fbo update code from cluter_texture_paint
  [texture] push/pop draw buffer when painting actors to a texture
  [texture] Avoid redundant use of cogl_clip_stack_save when drawing offscreen
  [cogl-draw-buffer] fix Cogl -> GL viewport coord conversion
  [cogl_clip_push_window_rect] fix Cogl -> GL coordinate conversion
  [matrix] Adds cogl_matrix_get_inverse API
  [debug] Adds a COGL_DEBUG=matrices debug option
  [cogl-matrix] Import Mesa's matrix manipulation code
  [cogl] avoid any state changes when cogl_set_backface_culling_enable is a nop
  [cogl] Use clockwise face winding for offscreen buffers with culling enabled
  ...