TextureTools: improve distance field computation without texelFetch().

GL 3.2 has texelFetch() and layout(pixel_center_integer), which means
that we integer coordinates with no precision loss when addressing
individual pixels in the source texture. In the versions before we have
to craft floating-point coordinates for texture() to grab the value of
wanted pixel with no jumping around or interpolation.

This change improves the behavior *a bit*, but not fully. I'm postponing
this to the point when I have an unit test that compares the output with
ground truth.

src/Magnum/TextureTools/DistanceFieldShader.frag

@@ -87,7 +87,7 @@ void main() {
     #ifdef TEXELFETCH_USABLE
     const mediump ivec2 position = ivec2(gl_FragCoord.xy*scaling);
     #else
-    const mediump vec2 position = gl_FragCoord.xy*scaling*imageSizeInverted;
+    const mediump vec2 position = (gl_FragCoord.xy - vec2(0.5))*scaling*imageSizeInverted;
     #endif
 
     /* If pixel at the position is inside (1), we are looking for nearest pixel
@@ -111,7 +111,7 @@ void main() {
             #ifdef TEXELFETCH_USABLE
             const lowp ivec2 offset = ivec2(-i+j, i);
             #else
-            const lowp vec2 pixelOffset = vec2(float(-i+j), float(i));
+            const lowp vec2 pixelOffset = vec2(float(-i+j), float(i)) + vec2(0.5);
             const lowp vec2 offset = pixelOffset*imageSizeInverted;
             #endif