UBO by index instead of map

[alexcristici]

I replaced StringIdentity idBackgroundDrawableUBOName with

enum {
    idBackgroundDrawableUBO,
    idBackgroundLayerUBO,
    backgroundUBOCount
};

UBOs ids are in the same order in UniformBlocks as in UniformBuffers, using above enum ordering.
In this way we match the uniform buffer for the right uniform block.

[github-actions]

Bloaty Results 🐋

Compared to main

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  -0.1%  -208Ki  -0.2% -54.7Ki    TOTAL

Full report: https://maplibre-native.s3.eu-central-1.amazonaws.com/bloaty-results/pr-1980-compared-to-main.txt

Compared to d387090 (legacy)

    FILE SIZE        VM SIZE    
 --------------  -------------- 
   +18% +21.2Mi  +398% +23.8Mi    TOTAL

Full report: https://maplibre-native.s3.eu-central-1.amazonaws.com/bloaty-results/pr-1980-compared-to-legacy.txt

[github-actions]

Bloaty Results (iOS) 🐋

Compared to main

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  -0.2% -36.0Ki  -0.2% -32.0Ki    TOTAL

Full report: https://maplibre-native.s3.eu-central-1.amazonaws.com/bloaty-results-ios/pr-1980-compared-to-main.txt

[mwilsnd]

Looking good! Guess I need to dust off that shader code generation PR when I have a chance and really update it for this.