Bug 1785947 - Fix Linux 32-bit WPT backdrop filter tests. r=gfx-reviewers,jrmuizel

In CI, we fail the glxtest which disables all gfx features, including
the backdrop filter feature. We can just enable it on Linux. A follow up
series of patches will reform the blocklist to prevent these footguns in
the future in the next nightly cycle.

Differential Revision: https://phabricator.services.mozilla.com/D155041

widget/gtk/GfxInfo.cpp

@@ -976,6 +976,11 @@ nsresult GfxInfo::GetFeatureStatusImpl(
 
   GetData();
 
+  if (aFeature == nsIGfxInfo::FEATURE_BACKDROP_FILTER) {
+    *aStatus = nsIGfxInfo::FEATURE_STATUS_OK;
+    return NS_OK;
+  }
+
   if (mGlxTestError) {
     // If glxtest failed, block all features by default.
     *aStatus = nsIGfxInfo::FEATURE_BLOCKED_DEVICE;