DebugTools, TextureTools: no pixel_center_integer in GLSL ES.

This seems like yet another pointless restriction. Aargh.

src/Magnum/DebugTools/TextureImage.frag

@@ -1,12 +1,18 @@
 uniform highp int level;
 uniform highp sampler2D textureData;
 
+#ifndef GL_ES
 layout(pixel_center_integer) in highp vec4 gl_FragCoord;
+#endif
 
 out highp uvec4 fragmentOutput;
 
 void main() {
+    #ifndef GL_ES
     ivec2 pos = ivec2(gl_FragCoord.xy);
+    #else
+    ivec2 pos = ivec2(gl_FragCoord.xy - vec2(0.5));
+    #endif
 
     fragmentOutput = floatBitsToUint(texelFetch(textureData, pos, level));
 }

src/Magnum/TextureTools/DistanceFieldShader.frag

@@ -52,7 +52,9 @@ uniform lowp sampler2D textureData;
 #endif
 
 #ifdef TEXELFETCH_USABLE
+#ifndef GL_ES
 layout(pixel_center_integer) in mediump vec4 gl_FragCoord;
+#endif
 #else
 #ifdef EXPLICIT_UNIFORM_LOCATION
 layout(location = 2) uniform vec2 imageSizeInverted;
@@ -85,8 +87,12 @@ bool hasValue(const mediump vec2 position, const mediump vec2 offset) {
 
 void main() {
     #ifdef TEXELFETCH_USABLE
+    #ifndef GL_ES
     const mediump ivec2 position = ivec2(gl_FragCoord.xy*scaling);
     #else
+    const mediump ivec2 position = ivec2((gl_FragCoord.xy - vec2(0.5))*scaling);
+    #endif
+    #else
     const mediump vec2 position = (gl_FragCoord.xy - vec2(0.5))*scaling*imageSizeInverted;
     #endif