TextureTools: don't require more than 8 texture units in distanceField().

ES2 contexts on iOS don't have more than that. It's weird, because ES3
contexts have 16.

doc/changelog.dox

@@ -122,6 +122,12 @@ See also:
     @ref Shaders::VertexColor::Color4 attribute specifiers for more convenient
     distinction between three- and four-component vertex color attribute.
 
+@subsubsection changelog-latest-changes-texturetools TextureTools library
+
+-   Fixed @ref TextureTools::distanceField() to not require more than 8 texture
+    units, which is the limit on all iOS ES2 contexts, independently on the
+    device (ES3 contexts have 16).
+
 @subsubsection changelog-latest-changes-trade Trade library
 
 -   @ref Trade::PhongMaterialData now contains well-defined color values

src/Magnum/TextureTools/DistanceField.cpp

@@ -75,7 +75,9 @@ class DistanceFieldShader: public GL::AbstractShaderProgram {
         }
 
     private:
-        enum: Int { TextureUnit = 8 };
+        /* ES2 on iOS (apparently independent on the device) has only 8 texture
+           units, so be careful to not step over that. ES3 has 16. */
+        enum: Int { TextureUnit = 7 };
 
         Int radiusUniform{0},
             scalingUniform{1},

src/Magnum/TextureTools/DistanceFieldShader.frag

@@ -46,7 +46,7 @@ uniform mediump vec2 scaling;
 #endif
 
 #ifdef EXPLICIT_TEXTURE_LAYER
-layout(binding = 8) uniform sampler2D textureData;
+layout(binding = 7) uniform sampler2D textureData;
 #else
 uniform lowp sampler2D textureData;
 #endif