6531: fix transparency in grass_main r=Jupeyy a=mindTW

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Closes  #6499.

before:
![grafik](https://user-images.githubusercontent.com/46008453/233847590-f8462f81-4525-449d-859c-18bfb07f53ff.png)

after:
![grafik](https://user-images.githubusercontent.com/46008453/233847608-211212ed-7fcd-46d3-a2b5-9ad92246f95a.png)


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## Checklist

- [x] Tested the change ingame
- [x] Provided screenshots if it is a visual change
- [ ] Tested in combination with possibly related configuration options
- [ ] Written a unit test (especially base/) or added coverage to integration test
- [ ] Considered possible null pointers and out of bounds array indexing
- [x] Changed no physics that affect existing maps
- [ ] Tested the change with [ASan+UBSan or valgrind's memcheck](https://github.com/ddnet/ddnet/#using-addresssanitizer--undefinedbehavioursanitizer-or-valgrinds-memcheck) (optional)


Co-authored-by: mindTW <mindteeworlds@gmail.com>
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bors[bot] 2023-04-23 19:10:53 +00:00 committed by GitHub
commit 4d39245e58
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