[Qt for WebAssembly; opengl-2 branch] Fix http_file_source

[birkskyum]

The manual changes required for qt webgl1 compilation are here fixed with compile conditionals:

https://github.com/birkskyum/maplibre-native-wasm/tree/main/qt-opengl2#changes-needed-to-the-code

(i'll make sure to clean up this branch - made it in main repo instead of fork by accident)

[ntadej]

This is still causing a leak in WASM build. As this is not urgent, let me try to find a proper fix.

[ntadej]

Let me also do a review.

[birkskyum]

This is still causing a leak in WASM build.

This actually closes a leak in WASM build by allowing the browser to abort tile requests. It's easy to verify, because without this fix the app stalls and crashes shortly after boot due to a lack of cleanup.

@ntadej Can we maybe merge this fix, so that things work out-of-the-box, and then at your convenience you can look into improving on it further if the opengl-2 branch still is relevant at the time?