- Try to reduce regenerations of reg_ignore. Highly experimental and
could lead to very obscure bugs. More testing needed.
- Introduce to_paint in struct _win to keep track of whether this window
was painted last time.
- Drop CAN_DO_USABLE support. Its usage looks pretty limited.
- Fix a bug that possibly causes rendering issues on frame width changes
if frame_opacity is enabled.
- Detect other borders (instead of only top border) when determining
frame opacity.
- Change the type of w->mode from int to an enumeration type.
- Ignore ShapeNotify if the window is not mapped, to avoid loss of
w->border_size in some cases, which breaks the fading out process of
shaped windows.
- Stop rendering a window if its picture is lost and it's unmapped, to
avoid a series of X errors and possible rendering problems.