eframe support for wgpu on the web

[Wumpf]

Adds a wgpu painter for eframe's web compilation (i.e. targeting wasm).
You can render the demo with it by replacing glow with wgpu in the feature list in build_demo_web.sh. Worked fine for me on manual testing!

Fixes #1755

Remaining things to figure out (likely after this PR):

• target is currently srgb, this causes blending to be done incorrectly. It seems wgpu doesn't support anything else for webgl canvases right now (why?)
• associate a depth buffer with the egui-wgpu renderer
• expose MSAA settings
• expose limits settings etc.
• allow targeting WebGPU

[Wumpf]

filed wgpu issue for srgb issue gfx-rs/wgpu#3059

[emilk]

crates/egui_demo_app/src/apps/custom3d_glow.rs should be made to work before we merge this (or disabled on wasm32+wgpu)

[Wumpf]

Fixed!

[emilk]

I get this warning in Firefox:

WebGL warning: getSyncParameter: ClientWaitSync must return TIMEOUT_EXPIRED until control has returned to the user agent's main loop. (only warns once)

No such warning on chromium.

[emilk]

This works great, good job! ⭐

[emilk]

The demo app WASM increases in size by a whopping 1.5 MB (4.7MB -> 6.3MB). Not great. wgpu needs to go on a diet!

[Wumpf]

I get this warning in Firefox:

WebGL warning: getSyncParameter: ClientWaitSync must return TIMEOUT_EXPIRED until control has returned to the user agent's main loop. (only warns once)

No such warning on chromium.

Saw it as well, filed an issue gfx-rs/wgpu#3064

[emilk]

So starting with this, eframe::start_web is now async. This, I believe, makes it incompatible with trunk, which eframe_template is using 😭

I opened this trunk issue: trunk-rs/trunk#466