In commit 10e58a4a I changed the code in cairo-surface.c to avoid
setting surface->is_clear = FALSE; in some situations where it was not
necessary, because the operation did not actually modify anything (it
returned CAIRO_INT_STATUS_NOTHING_TO_DO). However, that change
accidentally also caused _cairo_surface_paint() not to set
surface->is_clear = TRUE; in similar cases. That was unintended.

This commit fixes that by always setting is_clear = TRUE when necessary,
but keeps the optimisation of not setting is_clear = FALSE when not
necessary.

The connection to the below issue is that the issue happened with
surfaces with width=0. Clearing such a surface with CAIRO_OPERATOR_CLEAR
causes CAIRO_INT_STATUS_NOTHING_TO_DO and thus is_clear = TRUE was not
set. This error was later caught by a failed assertion.

Fixes: cairo/cairo#283


Signed-off-by: Uli Schlachter <psychon@znc.in>