Clean up gdk_draw_drawable() composite handling

Clean up gdk_draw_drawable() composite handling

Instead of doing some magic in gdk_draw_drawable() to avoid double
offsetting when calling gdk_draw_drawable on the impl we call
the vfunc directly on the impl. Thus removing the weird magic from
gdk_draw_drawable().

I tested this with the testgtk test "text", where if the original magic
code is disabled typing a newline in the middle of a text line causes
the double offset issue to appear.