X11Window: Reduce set_viewport() calls to reduce resize flicker corruption

[keigen-shu]

This is a workaround to reduce flickering corruption that happens when resizing the window. The changes are as follows:

• When polling X11 ConfigureNotify events, only the last resize event will be used to set the new window size. The entire viewport is then repainted.
• X11 Expose events now cause an immediate redraw instead of being cached in a buffer.
• When requesting a repaint, X11 Expose events are no longer created by ClanLib and then looped back over X11.
• Removed undocumented class members that looked redundant.

Note: It seems that window flickering when resizing very common problem; even glxgearshas flickering problems when resizing. Most sources online recommend always re-drawing the entire screen when receiving the X11 Expose event (rather buffering them first) which implemented in the last commit.

[keigen-shu]

Closing because there are still some problems relating to actual window sizing (after X11 borders and WM added frame extents) on X11Window. Will re-open once I go through all of it.

[rombust]

Requested to close