Cogl now has public experimental API to create a rectangle texture
which we can use instead of creating a foreign texture with GL. This
avoids Mutter depending on Cogl including a GL header from its public
headers which it might not do in future.
https://bugzilla.gnome.org/show_bug.cgi?id=672711
meta-texture-rectangle and meta-shaped-texture both create textures
with GL_TEXTURE_RECTANGLE_ARB as the target using direct GL
calls. This patch moves that code into a shared utility function in a
separate file instead. The function resolves the required GL symbols
dynamically instead of linking to them directly so that if Clutter
eventually stops linking to -lGL mutter will continue to build. The
function also splits the texture creation into a separate texture
creation and data upload stage so that it can use
cogl_texture_set_region to upload the data. That way it can avoid
clobbering the glPixelStore state and it can let Cogl do any necessary
format conversion. The code preserves the old value of the rectangle
texture binding instead of clobbering it because Cogl expects to be
able to cache this value to avoid redundant glBindTexture
calls. Finally, the function uses cogl_object_set_data to
automatically destroy the GL texture when the Cogl texture is
destroyed. This avoids having to have special code to destroy the cogl
texture.
https://bugzilla.gnome.org/show_bug.cgi?id=654569
