Increased precision in shader example

On BB10 the shader effect displayed as a black rectangle.

Change-Id: I9cd12e66875c07cc7ecfa2aec6d7e5912926511e
Reviewed-by: Alan Alpert <aalpert@blackberry.com>
Reviewed-by: Paul Olav Tvete <paul.tvete@digia.com>

examples/quick/shadereffects/shadereffects.qml

@@ -223,8 +223,8 @@ Rectangle {
                     lowp vec4 tr = texture2D(source, qt_TexCoord0 + vec2(delta.x, -delta.y));
                     lowp vec4 bl = texture2D(source, qt_TexCoord0 - vec2(delta.x, -delta.y));
                     lowp vec4 br = texture2D(source, qt_TexCoord0 + delta);
-                    lowp vec4 gx = (tl + bl) - (tr + br);
-                    lowp vec4 gy = (tl + tr) - (bl + br);
+                    mediump vec4 gx = (tl + bl) - (tr + br);
+                    mediump vec4 gy = (tl + tr) - (bl + br);
                     gl_FragColor.xyz = vec3(0.);
                     gl_FragColor.w = clamp(dot(sqrt(gx * gx + gy * gy), vec4(1.)), 0., 1.) * qt_Opacity;
                 }"