Wgpu 23 (GLES) no longer works on devices with max_color_attachments == 4.

[adrian17]

Description
On v22 and before, wgpu initialized on devices with max_color_attachments == 4. This includes some Android and Windows devices, and possibly some browsers with WebGL2. On v23 and above, it fails with Limit 'max_color_attachments' value 8 is better than allowed 4.

Repro steps
If you don't happen to have such a device, you can use JS to easily pretend, just enter this snippet in devtools console before the wgpu application with webgl backend initializes:

var temp = WebGL2RenderingContext.prototype.getParameter; WebGL2RenderingContext.prototype.getParameter = function(n){    if (n == 0x8CDF) { return 4; } return temp.call(this, n);  }

Expected vs observed behavior
Making it possible to support these devices again. AFAIK, dropping support for them was not documented in changelog.

If it turns out that the limit was always supposed to be 8 and allowing these devices to use wgpu was an accident, then we'd still appreciate some optional way to drop it back to 4 - since we don't even need 4 for our use cases AFAIK.

[adrian17]

For the record: we use wgpu::Limits::downlevel_webgl2_defaults. They are documented as

These default limits are guaranteed to be compatible with GLES-3.0, and D3D11, and WebGL2

But the GLES3 minimum for MAX_COLOR_ATTACHMENTS is 4.

[cwfitzgerald]

Sounds like the downlevel limits need to be reduced to 4 - can you submit a PR? This limit was previously not enforced

[adrian17]

I can, sure. The only caveat is that we don't have access to any real android devices with this issue, so I can't prove that this is the only thing needed to fix; we might only realize this some time after release.

[teoxoy]

Related: #3543.