port drawing fill-extrusions directly into the main framebuffer

[ansis]

Fill-extrusions are currently drawing into a offscreen texture before being copied into the main framebuffer.

In mapbox/mapbox-gl-js#7821 -js switched to drawing fill-extrusions directly into the main framebuffer instead of drawing to an offscreen texture first. This was done for reasons that don’t apply to -native (antialiasing for custom layers) but it also improves performance by avoiding a full screen copy.

Most of these changes need to be ported to -native.

For a high level description of the changes see mapbox/mapbox-gl-js#7821

The main changes that need to be ported are:

• drawing buildings directly into the main framebuffer
• drawing semi-transparent buildings with two passes (depth only for the first pass, shading in the second pass)
• only using the opaque pass for fills that are below all buildings
• (maybe) optimizing stencil buffer usage

Let me know when someone picks this up! I can help explain things further

[kkaefer]

Porting this blocks GL JS module updates in GL native due to shader changes.

[tristen]

Just flagging: I opened a ticket that this issue addresses in mapbox/mapbox-gl-js#8124. It's fixed in 0.54.0-beta.1 of Mapbox GL JS. On mobile, a fill-extrusion layer ignores the ordering of a fill layer ordered above it.

Preview

Other geometries (like the white line string) is fine.

[kkaefer]

This was merged in #14526