Fix precision issue in HDR upsampling shader.

Author: kenrussell

hdr/hdr.html

@@ -218,7 +218,7 @@
 <script id="bicubicUpsamplingVertexShader" type="text/something-not-javascript">
 attribute vec4 position;
 varying vec2 v_texCoord;
-uniform vec2 u_imageIncrement;
+uniform mediump vec2 u_imageIncrement;
 void main() {
   v_texCoord = (position.xy + 1.0) * 0.5 - vec2(0.5, 0.5) * u_imageIncrement;
   gl_Position = position;