Custom drawable layer: Simple Fill

[stefankarschti]

feat: Add a simple fill to the capabilities of the custom drawable layer's interface.

Changes:

• a Fill Generator helper has been created in mbgl::gfx. This will generate vertex, index buffers and segments compatible with the Fill shaders.
• The FillBucket::addFeature method now uses the above Fill buffer Generator too
• The CustomDrawableLayerHost::Interface has new methods added to manage addition of fill geometries: setFillOptions and addFill
• platform/darwin/app/ExampleCustomDrawableStyleLayer.mm has been updated to show example usage of the new fill capability. Instantiation is done from the addCustomDrawableLayer method in MBXViewController.m.
• The API test CustomDrawableLayer.Basic has been updated to include Fill

Refs:#1735

[github-actions]

Bloaty Results 🐋

Compared to main

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +0.2%  +247Ki  +0.2% +63.6Ki    TOTAL

Full report: https://maplibre-native.s3.eu-central-1.amazonaws.com/bloaty-results/pr-1810-compared-to-main.txt

Compared to d387090 (legacy)

    FILE SIZE        VM SIZE    
 --------------  -------------- 
   +19% +22.3Mi  +402% +24.0Mi    TOTAL

Full report: https://maplibre-native.s3.eu-central-1.amazonaws.com/bloaty-results/pr-1810-compared-to-legacy.txt

[sjg-wdw]

I like it!

[louwers]

Could you write a short summary of the changes you had to make in the PR comment?