cogl: Make CoglSubTexture only work for quad rendering

The sub texture backend doesn't work well as a completely general
texture backend because for example when rendering with cogl_polygon
it needs to be able to tranform arbitrary texture coordinates without
reference to the other coordintes. This can't be done when the texture
coordinates are a multiple of one because sometimes the coordinate
should represent the left or top edge and sometimes it should
represent the bottom or top edge. For example if the s coordinates are
0 and 1 then 1 represents the right edge but if they are 1 and 2 then
1 represents the left edge.

Instead the sub-textures are now documented not to support coordinates
outside the range [0,1]. The coordinates for the sub-region are now
represented as integers as this helps avoid rounding issues. The
region can no longer be a super-region of the texture as this
simplifies the code quite a lot.

There are two new texture virtual functions:

transform_quad_coords_to_gl - This transforms two pairs of coordinates
     representing a quad. It will return FALSE if the coordinates can
     not be transformed. The sub texture backend uses this to detect
     coordinates that require repeating which causes cogl-primitives
     to use manual repeating.

ensure_non_quad_rendering - This is used in cogl_polygon and
     cogl_vertex_buffer to inform the texture backend that
     transform_quad_to_gl is going to be used. The atlas backend
     migrates the texture out of the atlas when it hits this.

cogl/cogl-texture-2d.c

@@ -342,6 +342,15 @@ _cogl_texture_2d_transform_coords_to_gl (CoglTexture *tex,
      anything */
 }
 
+static gboolean
+_cogl_texture_2d_transform_quad_coords_to_gl (CoglTexture *tex,
+                                              float *coords)
+{
+  /* The texture coordinates map directly so we don't need to do
+     anything */
+  return TRUE;
+}
+
 static gboolean
 _cogl_texture_2d_get_gl_texture (CoglTexture *tex,
                                  GLuint *out_gl_handle,
@@ -399,6 +408,12 @@ _cogl_texture_2d_ensure_mipmaps (CoglTexture *tex)
   tex_2d->mipmaps_dirty = FALSE;
 }
 
+static void
+_cogl_texture_2d_ensure_non_quad_rendering (CoglTexture *tex)
+{
+  /* Nothing needs to be done */
+}
+
 static gboolean
 _cogl_texture_2d_set_region (CoglTexture    *tex,
                              int             src_x,
@@ -612,9 +627,11 @@ cogl_texture_2d_vtable =
     _cogl_texture_2d_is_sliced,
     _cogl_texture_2d_can_hardware_repeat,
     _cogl_texture_2d_transform_coords_to_gl,
+    _cogl_texture_2d_transform_quad_coords_to_gl,
     _cogl_texture_2d_get_gl_texture,
     _cogl_texture_2d_set_filters,
     _cogl_texture_2d_ensure_mipmaps,
+    _cogl_texture_2d_ensure_non_quad_rendering,
     _cogl_texture_2d_set_wrap_mode_parameter,
     _cogl_texture_2d_get_format,
     _cogl_texture_2d_get_gl_format,