Auto merge of #24128 - asajeffrey:webgl-extensions-fallback, r=Manishearth

Fallback to old extensions API if NUM_EXTENSIONS errors

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Fixes a panic querying extensions in WebGL 1.

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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #21993
- [x] These changes do not require tests because it fixes a panic

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components/canvas/webgl_thread.rs

@@ -1727,6 +1727,11 @@ impl WebGLImpl {
         unsafe {
             gl.get_integer_v(gl::NUM_EXTENSIONS, &mut ext_count);
         }
+        // Fall back to the depricated extensions API if that fails
+        if gl.get_error() != gl::NO_ERROR {
+            chan.send(gl.get_string(gl::EXTENSIONS)).unwrap();
+            return;
+        }
         let ext_count = ext_count[0] as usize;
         let mut extensions = Vec::with_capacity(ext_count);
         for idx in 0..ext_count {