Bug 1645124 - Bump the number of layers per alpha texture array from 4 to 8. r=gw

This prevent frequent batch breaks with non-sub-px-aa text runs.

Differential Revision: https://phabricator.services.mozilla.com/D79327

gfx/wr/webrender/src/texture_cache.rs

@@ -239,7 +239,7 @@ impl SharedTextures {
             array_alpha8_linear: TextureArray::new(
                 TextureFormatPair::from(ImageFormat::R8),
                 TextureFilter::Linear,
-                4,
+                8,
             ),
             // Used for experimental hdr yuv texture support, but not used in
             // production Firefox.