fix: set width and height for vjs-button like the SubsCaps button (#4548)

Add a width and height property of 100% to `.vjs-control .vjs-button` selector.

Fixes #4547.

src/css/components/_button.scss

@@ -15,3 +15,8 @@
   -moz-appearance:    none;
   appearance:         none;
 }
+
+.vjs-control .vjs-button {
+  width: 100%;
+  height: 100%;
+}