[core] Refactor wireframe to match JS overdraw

[brunoabinader]

This PR:

• Adds a new BlendColor GL configuration value
• Generates an additional overdraw fragment shader & program for each shader type
• Refactors the wireframe mode to behave like JS overdraw mode, where each render pass over the same place increases the color by 1/8 from black to white.

Fixes #5371.

Snapshot:

Notes:

• Although similar, it seems that Wireframe and Overdraw serves different purposes - we could add a new debug mode instead of replacing wireframe implementation.
• Do we really need to overdraw SDFs?

👀 @mapbox/gl

[brunoabinader]

I suspect the debug/overdraw render test differs between -js and -native because in -native we optimize the background layer via glClear. However, I'm yet to find a proper solution for that.

[brunoabinader]

When 1) disabling the glClear optimization for the bottommost solid background layer and 2) not resetting the blend function in the translucent render pass we are now getting 1:1 overdraw results between -js and -native 🎉:

[kkaefer]

Awesome!

[friedbunny]

I initially thought this would be good to cherry-pick into the release-ios-v3.3.0 branch, but it includes too many intervening core changes for that to be tenable.

[1ec5]

This enum value needs a more descriptive name, and the comment is somewhat nonsensical to an iOS developer.

[1ec5]

@friedbunny, @brunoabinader renamed the public enum value, so we either need to revert the Objective-C changes or backport them to the release branch. Otherwise, the release after 3.3.0 will break backwards compatibility.

[friedbunny]

@1ec5 I suppose just porting the Obj-C changes without including the refactored implementation itself is a thing we can do. Functionality would change after v3.3.0, but the API would remain the same.