enforce discard only in fragment

[Uriopass]

Fixes #2263

as per wgsl spec "The discard statement must only be used in a fragment shader stage."

[codecov-commenter]

Codecov Report

Merging #2262 (15dc8c9) into master (00be08e) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 15dc8c9 differs from pull request most recent head 112a618. Consider uploading reports for the commit 112a618 to get more accurate results

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@@           Coverage Diff           @@
##           master    gfx-rs/wgpu#2262   +/-   ##
=======================================
  Coverage   82.05%   82.05%           
=======================================
  Files          80       80           
  Lines       43241    43242    +1     
=======================================
+ Hits        35481    35484    +3     
+ Misses       7760     7758    -2     

Impacted Files	Coverage Δ
src/valid/function.rs	74.09% <100.00%> (+0.04%)	⬆️
src/valid/analyzer.rs	95.22% <0.00%> (+0.12%)	⬆️
src/valid/interface.rs	77.25% <0.00%> (+0.23%)	⬆️

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[teoxoy]

Looks good! Thank you for the PR!