It should be possible render a single PangoLayout with different
colors without recalculating the layout. This was not working because
the color used at the first edit was being stored in the display
list. This broke changing the opacity on a ClutterText.
Now each node in the display list has a 'color override' flag which
marks whether it should use the base color or not. The base color is
now passed in from _cogl_pango_display_list_render_texture. The alpha
value is always taken from the base color.
When rendering a pango layout CoglPangoRenderer now records the
operations into a list called a CoglPangoDisplayList. The entries in
the list are either glyph renderings from a texture, rectangles or
trapezoids. Multiple consecutive glyph renderings from the same glyph
cache texture are combined into a single entry. Note the
CoglPangoDisplayList has nothing to do with a GL display list.
After the display list is built it is attached to the PangoLayout with
g_object_set_qdata so that next time the layout is rendered it can
bypass rebuilding it.
The glyph rendering entries are drawn via a VBO. The VBO is attached
to the display list so it can be used multiple times. This makes the
common case of rendering a PangoLayout contained in a single texture
subsequent times usually boil down to a single call to glDrawArrays
with an already uploaded VBO.
The VBOs are accessed via the cogl_vertex_buffer API so if VBOs are
not available in GL it will resort to a fallback.
Note this will fall apart if the pango layout is altered after the
first render. I can't find a way to detect when the layout is
altered. However this won't affect ClutterText because it creates a
new PangoLayout whenever any properties are changed.
