Bug 1624640 - Grow the alpha8 texture array four layers at a time. r=jrmuizel

The texture array is currently grown layer by layer and we typically get to 3 or 4 layers over several frames by the time we are done loading a simple wikipedia page.

Differential Revision: https://phabricator.services.mozilla.com/D68056

--HG--
extra : moz-landing-system : lando

gfx/wr/webrender/src/texture_cache.rs

@@ -280,7 +280,7 @@ impl SharedTextures {
             array_alpha8_linear: TextureArray::new(
                 TextureFormatPair::from(ImageFormat::R8),
                 TextureFilter::Linear,
-                1,
+                4,
             ),
             // Used for experimental hdr yuv texture support, but not used in
             // production Firefox.