tests: fix pixel coordinate test using too low precision in opengl es

testing/tests/graphics.lua

@@ -211,8 +211,8 @@ love.test.graphics.Canvas = function(test)
     vec4 effect(vec4 c, Image tex, vec2 tc, vec2 pc) {
       // rounding during quantization from float to unorm8 doesn't seem to be
       // totally consistent across devices, lets do it ourselves.
-      vec2 value = pc / love_ScreenSize.xy;
-      vec2 quantized = (floor(255.0 * value + 0.5) + 0.1) / 255.0;
+      highp vec2 value = pc / love_ScreenSize.xy;
+      highp vec2 quantized = (floor(255.0 * value + 0.5) + 0.1) / 255.0;
       return vec4(quantized, 0.0, 1.0);
     }
   ]]