WASM: Fix glyph transparency on GL

[chemicstry]

This PR fixes UI glyph rendering on GL. Previously glyph data was provided as R8Unorm and swizzled into alpha channel, however, GL backend (and dx11 if it's ever used) does not support swizzling. So we build the full texture instead. I'm not sure if this could cause performance problems? This also makes #1698 obsolete, so I removed redudant shader attributes.

Tested on GL and vulkan, but not on metal (would appreciate someone testing UI examples).

Although vulkan complains about image layout, but this already existed before the changes. I was unable to fix it.

[WARN][gfx_backend_vulkan] 
PERFORMANCE [UNASSIGNED-CoreValidation-DrawState-InvalidImageLayout (0)] : vkCmdCopyBufferToImage(): For optimal performance VkNonDispatchableHandle 0xde3f1a000000001f[] layout should be VK_IMAGE_LAYOUT_TRANSFER_DST_OPTIMAL instead of GENERAL.
object info: (type: COMMAND_BUFFER, hndl: 219292848)

Part of #2192

[codecov]

Codecov Report

Merging #2217 into wasm will not change coverage by %.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             wasm    #2217   +/-   ##
=======================================
  Coverage   83.14%   83.14%           
=======================================
  Files          74       74           
  Lines        6088     6088           
=======================================
  Hits         5062     5062           
  Misses       1026     1026           

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[chemicstry]

Did some profiling on performance. Ran states_ui example and measured execution times of 2 glyph rendering batches (one to render menu and other to render demo screen).

Before this PR: 43.37 ms and 175.43 ms
After this PR: 47.59 ms and 184.59 ms

So there is some performance hit. Alternative would be to cfg everything into GL and non-GL, but in my opinion that would really affect readability of the code. Any thoughts?

On a side note, I looked into implementing swizzling in gfx, but their API abstractions make it really hard for GL backend.

[azriel91]

✌️

[azriel91]

bors r+

[bors]

Build succeeded

• continuous-integration/jenkins/branch