Bug 1526769 - always initialize local_transform to the identity matri…

…x r=nical

[wrupdater] From #3547

Differential Revision: https://phabricator.services.mozilla.com/D19305

[wrupdater] From https://hg.mozilla.org/mozilla-central/rev/0d65ca0a00195e03b4b7976d6bbbf5c5cb0c9d93

webrender/res/ps_text_run.glsl

@@ -71,7 +71,7 @@ VertexInfo write_text_vertex(RectWithSize local_clip_rect,
                              vec2 snap_bias) {
     // The offset to snap the glyph rect to a device pixel
     vec2 snap_offset = vec2(0.0);
-    mat2 local_transform;
+    mat2 local_transform = mat2(1.0);
 
 #ifdef WR_FEATURE_GLYPH_TRANSFORM
     bool remove_subpx_offset = true;