Shader: Refine preprocessing on GLES platform

Don't globally set float precision to mediump, only fragment
shaders need that and defining it for vertex shaders causes
tilemap cracks.

Also manually define low precision for variables that hold
color / alpha values.

shader/plane.frag

@@ -1,11 +1,11 @@
 
 uniform sampler2D texture;
 
-uniform vec4 tone;
+uniform lowp vec4 tone;
 
-uniform float opacity;
-uniform vec4 color;
-uniform vec4 flash;
+uniform lowp float opacity;
+uniform lowp vec4 color;
+uniform lowp vec4 flash;
 
 varying vec2 v_texCoord;