Automatically select minimum limits for a specific platform

[Ciubix8513]

Automatically select minimum limits for a specific platform

[cwfitzgerald]

Additionally, the WebGL limits for texture size are so low (2k), that it's very common to crash when creating a surface because people have 1440p/4k monitors regularly. The only way to really use downlevel_defaults_webgl2 is with .with_resolution(adapter.limits()) so that the texture size limits are actually large enough.

[Wumpf]

I understand why that would be move convenient, but with wgpu being a WebGPU (with benefits) abstraction first I think it makes a lot of sense that any backend that is not fully compilant with WebGPU is opt-in.
If the DeviceDescriptor would use the downlevel webgl features it would mean that by default you get a device that doesn't work with WebGPU limits which is much more confusing for what I regard as the much more common usecase.
The rationale for the different defaults that you can pick from (!) for Limits is further detailed here: the "true default" (Default::default) is defined as the WebGPU guaranteed set of features.