frontend: Improve preview source snapping

[Warchamp7]

Description

This PR makes a number of changes to source snapping in the preview:

• Sources no longer snap to hidden sources
• Edges can snap to any same axis edge of other sources
  • (The top edge of a source can snap to the top edge of a source)
• Sources snap along the entire axis
  • (Sources don't have to be touching to snap)
• Visual indicators for snapping
• Replaces calculation macro with a static method

Additionally

• Hidden sources can no longer be hovered or clicked in the preview to select them

obs64_wuvM7eemGG.mp4

Motivation and Context

I want to improve source snapping behaviour and visual feedback

How Has This Been Tested?

Moved a bunch of sources around. Toggles relevant snapping settings on and off to ensure behaviour matched settings.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

• Tweak (non-breaking change to improve existing functionality)

• Code cleanup (non-breaking change which makes code smaller or more readable)

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.

[PatTheMav]

Functionally this PR is fine, but it seems very brute force and not particularly efficient to me and falls victim to "featuritis", just piling on little features one by one, each doing its own thing in isolation with no regard to other (similar) features and the architecture behind it.

Preview control drawing should be a single process, by a single responsible entity in code that knows about all of its "features" and thus set up a single render pass to draw all the pieces it knows should be visible for any given frame.