Base instance semantics on GLES backend

[kvark]

Is your feature request related to a problem? Please describe.
GLES3 doesn't support the base instance natively. We work around it by re-binding the instance-rate vertex buffers and applying the offset. But this doesn't affect gl_InstanceIndex, which also needs to be adjusted.

Describe the solution you'd like
We'd need Naga to detect the use of "instance_index" semantics and emit a uniform uint _base_instance" uniform. Then for any access of "instance_index" we'd append this value of "_base_instance.
hal/gles needs to detect this and provide the value for this uniform whenever a draw call requires it.

Describe alternatives you've considered
Adding a downlevel flag for the base instance and not bothering to fix it.

Additional context
Follows #1510

[cwfitzgerald]

Superseded by #4276