css: reduce overdraws from "textview > text"

This was causing us to draw the same background content twice which is a
significant amount of bits to flip in the GPU for maximized windows,
especially twice.

gtk/theme/Default/_common.scss

@@ -66,6 +66,8 @@ textview {
   > text {
     @extend %view;
 
+    background-color: transparent;
+
     > selection {
       background-color: $backdrop_selected_bg_color;