scroll-view-fade: Use GLSL 1.20

GLSL 1.20 is a better language, and we'll rely on it in future updates.
This doesn't have any additional constraints, since GLSL 1.20 was
standardized before GLSL-supporting drivers came out.

https://bugzilla.gnome.org/show_bug.cgi?id=689249

src/st/st-scroll-view-fade.glsl

@@ -17,6 +17,8 @@
  * along with this program. If not, see <http://www.gnu.org/licenses/>.
  */
 
+#version 120
+
 uniform sampler2D tex;
 uniform float height;
 uniform float width;