Explicitly set heatmap stencil mode

[lbud]

Fixes #6806. We had not been setting the stencil mode explicitly in the heatmap texture copy pass, meaning it inherited the stencil mode from whatever the previous layer was — presumably heatmaps have been most used immediately above symbol layers (which disable stencil testing) but when they follow a stencil-enabled layer (e.g. fill layers) they inherit a single (most recently used) tile mask from that layer.

Launch Checklist

• briefly describe the changes in this PR
• write tests for all new functionality
• manually test the debug page
• @mourner how did you generate the alternate images for heatmap render tests? Or I suppose I could also just set a higher diff threshold here, since failing tests look very different: …?
  

[mourner]

how did you generate the alternate images for heatmap render tests? Or I suppose I could also just set a higher diff threshold here, since failing tests look very different: …?

@lbud hmm, not sure I follow... Alternate images?

[lbud]

@mourner it looks like many of the heatmap tests have two possible expected images depending on whether the platform supports half-float-textures-based rendering as of d61850a — is that something I need to worry about/should expect to be an issue for some gl-native platforms when this is ported?

[mourner]

@lbud the non-half-float versions were generated with GL JS render tests because headless gl doesn't support the half-float extension, and the half-float versions were generated with GL Native render tests locally. Ideally there would be two versions for every test that has heatmaps, but a higher diff is probably OK too.