Use the same approach we take for recoloring in GtkIconHelper now.
As part of this change, GtkCsSImageRecolor is changed to not derive
from GtkCssImageUrl anymore.
This is just lots of renaming.
The interface remains private, so the public API does not change, apart
from removing the definition of the Interface object to avoid
subclassing.
Instead of spamming stderr with g_warning, use the new
emit_error method of the GtkStyleProviderPrivate interface
to emit an error if loading an image fails.
We should not hardcode a scale of 1, this leads to
pixellated upscaled images at scale=2, even if the source
is an svg. By passing the proper scale, we can load the
svg at the correct size.
If we fail to load the image for a -gtk-recolor() expression,
fall back to using the image-missing icon instead of crashing,
and include more details in the warning message.
