MEGA65/CORE: window size configuration anomalies

[lgblgblgb]

Emulation window size ("zoom") should be settable by configuration and/or UI menu. However there is no clear/tested way to do that in the Xemu framework, and especially problematic in the MEGA65 emulator. Also UI does not reflect the current setting.

Old bug report: #246

Since the problem is more general than that, let's continue with this one instead.

Problems:

• Trying to implement zoom factor causes very strange effects (at least in the MEGA65 emulator): texture remains the same size in bigger window, or snapping to some strange size (xemu_window_snap_to_optimal_size problem in emutools.c?)
• MEGA65 emulator should/try reflect the current setting in menu
• CLI/config option should be presented for setting zoom factor automatically
• Interestingly, resizing the window with mouse, seems not to have any strange issue

I want:

• Configurable "zoom" (scale) factor: from CLI, config file (saveable option), and UI menu
• ~Store window position on demand / restore~ [DONE]
• Free "resize by hand" may go, it seems that feature is not even widely known or used, but causing problems!
• Of course, avoid visual glitches and things everywhere