gles: allow clearing index buffers

[grovesNL]

Connections
Noticed while testing WebGL manually, noticed it was referenced by some other projects too, e.g. iced-rs/iced#1241

Description
On some browsers like Chrome, it seems like ELEMENT_ARRAY_BUFFER can't be used as a destination for copy_buffer_sub_data if the source is COPY_READ_BUFFER, so we can't copy from zero_buffer. Instead we workaround this by uploading zeroes.

We could also consider caching a Vec filled with zeroes if we'd like to avoid an allocation, but it might result in several uploads depending on the size of the cached Vec.

Testing
Tested manually on my own project and the warning disappears. We should probably check it against all wgpu WebGL examples though.

[grovesNL]

I've tested the existing WebGL examples and they seem ok.

[cwfitzgerald]

We've committed worse sins

[expenses]

Just saw this! I've been dealing with trying to get index buffer to index buffer copies to work. I suppose you could implement that by mapping the source buffer memory and doing buffer_sub_data_u8_slice from that right? It'd be pretty (very?) slow, but would at least be implemented.

[expenses]

Just saw this! I've been dealing with trying to get index buffer to index buffer copies to work. I suppose you could implement that by mapping the source buffer memory and doing buffer_sub_data_u8_slice from that right? It'd be pretty (very?) slow, but would at least be implemented.

Hmm, it seems I don't actually need to do this. NVM!