Fix non-native full screen misc crash, background color, and transparency issues

[ychin]

Fixed misc bugs:

• Previously, setting a colorscheme while in non-native full screen would override the background color specified in 'fuoptions' (default is black). It also wouldn't update the specified color if it's specified to a highlight group (e.g. set fuopts=background:Normal).

• Transparency and background colors and setting colorscheme also didn't work well togehter. Changing a background color while in fullscreen would reset transparency back to 0.

Also, 'fuoptions' can now specify background color in 6-digit hex number now, in addition to the old 8-digit format. Also update docs to elaborate that when using 8-digit, the alpha component is ignored (since we use the 'transparency' setting instead).

Add new tests to test 'fuoptions' work.

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Also include the following commits:

Fix non-native full screen bad interaction with window delegate

This issue was detected when adding new tests for non-native full screen. When setting 'fuoptions' to empty and going full screen, the code would fail but also crash, due to a number of issues:

1. enterFullScreen's setting of presentationOptions somehow triggers a window resize on the normal window, which leads to windowDidResize delegate being called. This is a degenerate situation as the window controller thinks we are already in full screen even though we haven't finished setting up yet, and in particular nonFuVimViewSize is still 0. This leads to the desired desired frame size being set incorrectly to 0.
2. constrainRows:columns:toSize does not sanity check the results when we have such degenerate small sizes, and end up calculating a desired rows to -1, which makes no sense. This leads to various wrong calculations.
3. MMCoreTextView loops through the grid using a size_t even though grid.rows is an int. This is a poor code practice in general and results in a crash as the loop comparison cast the -1 to size_t and the loop didn't correctly terminate.

Fix 1 (the root cause) by making sure we detact the window delegate immediately when entering full screen to prevent any stray window messages from causing issues. Also safeguard windowDidResize so it only handles the full screen path if we have fullScreenEnabled set.

For 2 and 3, add the sanity checks and also fix the size_t to use int when looping.

For some reason this issue only showed up in CI but not in local testing, probably due to different screen environments.

Add tests for full screen code

Also, add some additional tests for full screen mode in general.

Basic tests to validate different full screen functionalities. Help prevents regressions such as #1515 where full screen simply stops working.

Not an exhaustive test for now. Some functionalities (e.g. multi-monitor, MacBook notch) are hard to mock up and validate in tests. Some other functionalities like restoring window size, delayed full screen on startup (when setting it from gvimrc) will be added later.

[ychin]

This actually fixes a MacVim free that could be easily reproduced if you have the following in the gvimrc:

set fuopt=
set fullscreen

The delayed full screen logic before would delay setting fullScreenEnabled, but it would still initialize fullScreenWindow. The new code checks for that before calculating the desired size in the windowDidResize callback which guards against this situation correctly.