GUI: Fix SoundFileView grid behavior

[mossheim]

This fixes #2033, an issue regarding the gridResolution and gridOffset behaviors of SoundFileView as well as a problem with selections not being visible.

Test code

(
w = Window.new("soundfile test", Rect(200, 300, 740, 100));
a = SoundFileView.new(w, Rect(20,20, 700, 60));
f = SoundFile.new;
f.openRead(Platform.resourceDir +/+ "sounds/a11wlk01.wav");

a.soundfile = f;
a.read(0, f.numFrames);
a.timeCursorOn_(true);
a.gridResolution = 2;
a.gridOffset = 0;
w.front;
)

// other resolution and offset values
a.gridResolution = 1;
a.gridResolution = 0.5;
a.gridOffset = 0.25;
a.gridOffset = 0.75;

// also try zooming with shift+mouse-drag

Previous behavior

• The leftmost grid block was only half revealed.
• The grid blocked the selection highlight.
• Setting gridResolution <= 1 did not work: the entire view would become the grid color.
• The grid resolution did not match the desired value.

New behavior

• Leftmost grid block is fully revealed.
• Selection highlight appears on top of grid.
• Setting gridResolution to any value is now supported.
• Grid resolution matches desired value.

I have also tested this to make sure that zoom and offset behavior is preserved.

Description of solution

The previous implementation had some incorrect math that I rewrote; see commit messages/code for details. I also fixed a few compiler warnings in the same file and changed the default selection color to something that I would call "better". Finally, I changed the documentation to correctly describe what gridOffset does; I noticed it was incorrect while working on this.

[nhthn]

lgtm. zooming is not working for me on linux, but that's also true in master. nvm zooming is fine

[mossheim]

could someone else take a look at this please? it's a really straightforward fix but there may be test cases I wasn't able to think of.

[patrickdupuis]

I tested this out using some sccode patches. It all works. I'm on linux as well though...

[mossheim]

Great thanks so much @patrickdupuis !