demos: Fix glarea demo on OpenGL/ES

Some implementations of the ES 1.00 shader (such as Google's ANGLE) do
not like the 'f' suffix for floats, so just drop it, as it should be
harmless to drop.

demos/gtk-demo/glarea-gles.fs.glsl

@@ -1,7 +1,7 @@
 precision highp float;
 
 void main() {
-  float lerpVal = gl_FragCoord.y / 500.0f;
+  float lerpVal = gl_FragCoord.y / 500.0;
 
-  gl_FragColor = mix(vec4(1.0f, 0.85f, 0.35f, 1.0f), vec4(0.2f, 0.2f, 0.2f, 1.0f), lerpVal);
+  gl_FragColor = mix(vec4(1.0, 0.85, 0.35, 1.0), vec4(0.2, 0.2, 0.2, 1.0), lerpVal);
 }