WebGl workaround for drawing single instance

[Wumpf]

Checklist

• Run cargo clippy.
• Run RUSTFLAGS=--cfg=web_sys_unstable_apis cargo clippy --target wasm32-unknown-unknown if applicable.
• Add change to CHANGELOG.md. See simple instructions inside file.

Connections
fixes #3578
backport: #3596

Description
Whenever there is any instance attribute, we need to need use an instanced draw call on ANGLE. Otherwise, the following error will be (potentially spuriously) generated and may render incorrectly:

[.WebGL-0000554C38753500] GL_INVALID_OPERATION: Error: 0x00000502, in ..\..\third_party\angle\src\libANGLE\renderer\d3d\VertexDataManager.cpp, reserveSpaceForAttrib:520. Internal error: 0x00000502: Vertex buffer is not big enough for the draw call.

Testing
Tested the backport to v0.15 on https://github.com/rerun-io/rerun

[grovesNL]

Thank you! I think we need this for both C::Draw and C::DrawIndexed, so we could probably remove the instance_count == 1/draw_arrays branch in C::Draw entirely

[Wumpf]

@grovesNL Given that Firefox just uses DrawArraysInstanced anyways in all circumstances (it's enabled for WebGL2 support which is a requirement for us), maybe we should simplify this entire PR down to just that and leave a comment? Is there any drawback to not always use the instanced version(s) in our hal layer?

Otherwise, only checking on is_angle sounds good! I thought this wouldn't work on the web correctly, but seems we get that information just fine.

[grovesNL]

maybe we should simplify this entire PR down to just that and leave a comment

That sounds good to me. I think originally we were considering these fall back paths whenever we don't have instancing support, but our minimum requirements have been raised a bit since then.

[Wumpf]

Done! Much simpler now, applied the same to the v0.15 backport :)

[grovesNL]

Thank you!