≥TH14: Don't crash if the selected fullscreen resolution exceeds the maximum screen resolution

[nmlgc]

This problem comes up very regularly in the #support channel on Discord, due to common laptop resolutions like 1366×768 or similar being smaller than the recommended 1280×960 resolution.

Could be solved in a number of ways that are better than crashing:
• Still running the game in 1280×960, but downscaling the rendered image to match the maximum resolution.
• Switching down to 960×720 or even 640×480 as necessary. (This is what windowed mode does.)
• Or just simply showing a clear error message.