gdk: Rename gdkpixbuf-drawable.c

There is nothing particularly specific to drawables
in there (and we don't have that concept anymore),
so just name the source file to match the header.
Easier for everybody.

gdk/meson.build

@@ -35,7 +35,7 @@ gdk_public_sources = files([
   'gdkmonitor.c',
   'gdkpaintable.c',
   'gdkpango.c',
-  'gdkpixbuf-drawable.c',
+  'gdkpixbuf.c',
   'gdkpipeiostream.c',
   'gdkrectangle.c',
   'gdkrgba.c',