Always use blending for basic fill layers

[mwilsnd]

Compared with the legacy renderer, we should always be using alpha blending for simple fill layers. I'm unable to replicate #2217 at the moment, unsure if this change will also address that issue.

Replicate with FB style, GLFW platform, press 'O' to toggle off online connectivity to permit sustained overzoom of fill layers which do overzoom color interpolation.

Before:

After

[github-actions]

Bloaty Results (iOS) 🐋

Compared to main

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  [ = ]       0  [ = ]       0    TOTAL

Full report: https://maplibre-native.s3.eu-central-1.amazonaws.com/bloaty-results-ios/pr-2296-compared-to-main.txt

[github-actions]

Bloaty Results 🐋

Compared to main

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  -0.0%    -432  -0.0%    -344    TOTAL

Full report: https://maplibre-native.s3.eu-central-1.amazonaws.com/bloaty-results/pr-2296-compared-to-main.txt

Compared to d387090 (legacy)

    FILE SIZE        VM SIZE    
 --------------  -------------- 
   +20% +23.3Mi  +406% +24.3Mi    TOTAL

Full report: https://maplibre-native.s3.eu-central-1.amazonaws.com/bloaty-results/pr-2296-compared-to-legacy.txt

[github-actions]

Benchmark Results ⚡

Benchmark                                                     Time             CPU      Time Old      Time New       CPU Old       CPU New
------------------------------------------------------------------------------------------------------------------------------------------
OVERALL_GEOMEAN                                            +0.0023         +0.0027             0             0             0             0

Full report: https://maplibre-native.s3.eu-central-1.amazonaws.com/benchmark-results/pr-2296-compared-to-main.txt

[TimSylvester]

Makes sense, if an alpha value is specified it's hard to imagine how we could get correct results without using it, but there is a clear intent to do that sometimes in legacy so I have a suspicion that there might be an even more obscure bug lurking there.

[louwers]

Seems to be fixed!!