Text: prevent minor DistanceFieldGlyphCacheGL differences on ES2.

And hint that in TextureTools::DistanceFieldGL docs as well.

src/Magnum/Text/DistanceFieldGlyphCacheGL.cpp

@@ -89,8 +89,10 @@ GlyphCacheFeatures DistanceFieldGlyphCacheGL::doFeatures() const {
 void DistanceFieldGlyphCacheGL::doSetImage(const Vector2i& offset, const ImageView2D& image) {
     GL::Texture2D input;
     input.setWrapping(GL::SamplerWrapping::ClampToEdge)
-        .setMinificationFilter(GL::SamplerFilter::Linear)
-        .setMagnificationFilter(GL::SamplerFilter::Linear);
+        /* Use nearest filter to avoid minor rounding errors on ES2 compared to
+           texelFetch() on ES3+ */
+        .setMinificationFilter(GL::SamplerFilter::Nearest, GL::SamplerMipmap::Base)
+        .setMagnificationFilter(GL::SamplerFilter::Nearest);
 
     /* The constructor already checked that the ratio is an integer multiple,
        so this division should lead to no information loss */

src/Magnum/TextureTools/DistanceFieldGL.h

@@ -155,6 +155,11 @@ class MAGNUM_TEXTURETOOLS_EXPORT DistanceFieldGL {
          * OpenGL ES) and @p rectangle size is expected to be a multiple of 2,
          * as that's what the generator shader relies on for correct pixel
          * addressing.
+         *
+         * @attention For consistent results on implementations that don't
+         *      support @glsl texelFetch() @ce and have rely on regular texture
+         *      filtering it's recommended to use @ref GL::SamplerFilter::Nearest
+         *      on the @p input.
          */
         #ifdef DOXYGEN_GENERATING_OUTPUT
         void operator()(GL::Texture2D& input, GL::Framebuffer& output, const Range2Di& rectangle, const Vector2i& imageSize