Fix DistanceFieldVector on WebGL iOS

src/Magnum/Shaders/AbstractVector.h

@@ -100,7 +100,11 @@ template<UnsignedInt dimensions> class AbstractVector: public GL::AbstractShader
     #else
     private:
     #endif
-        enum: Int { VectorTextureLayer = 15 };
+        /* Those textures are quite specific (and likely reused multiple times
+           per frame for e.g. text rendering, so put them in a specific slot.
+           Older iOS (and iOS WebGL) has only 8 texture binding slots, so can't
+           go above that. Binding 7 is used by TextureTools::DistanceField. */
+        enum: Int { VectorTextureLayer = 6 };
 
         explicit AbstractVector(NoCreateT) noexcept: GL::AbstractShaderProgram{NoCreate} {}
         explicit AbstractVector() = default;

src/Magnum/Shaders/DistanceFieldVector.frag

@@ -62,7 +62,8 @@ uniform lowp float smoothness
     ;
 
 #ifdef EXPLICIT_TEXTURE_LAYER
-layout(binding = 15)
+/* See AbstractVector.h for details about the ID */
+layout(binding = 6)
 #endif
 uniform lowp sampler2D vectorTexture;
 
@@ -83,8 +84,11 @@ void main() {
 
     /* Outline */
     if(outlineRange.x > outlineRange.y) {
-        lowp float mid = (outlineRange.x + outlineRange.y)/2.0;
+        /* Doing *0.5 instead of /2.0 because the latter causes iOS / WebGL to
-        lowp float halfRange = (outlineRange.x - outlineRange.y)/2.0;
+           complain that "Overflow in implicit constant conversion, minimum
+           range for lowp float is (-2,2)" */
+        lowp float mid = (outlineRange.x + outlineRange.y)*0.5;
+        lowp float halfRange = (outlineRange.x - outlineRange.y)*0.5;
         fragmentColor += smoothstep(halfRange+smoothness, halfRange-smoothness, distance(mid, intensity))*outlineColor;
     }
 }

src/Magnum/Shaders/Vector.frag

@@ -44,7 +44,8 @@ uniform lowp vec4 color
     ;
 
 #ifdef EXPLICIT_TEXTURE_LAYER
-layout(binding = 15)
+/* See AbstractVector.h for details about the ID */
+layout(binding = 6)
 #endif
 uniform lowp sampler2D vectorTexture;
 

src/Magnum/TextureTools/DistanceField.cpp

@@ -73,7 +73,9 @@ class DistanceFieldShader: public GL::AbstractShaderProgram {
 
     private:
         /* ES2 on iOS (apparently independent on the device) has only 8 texture
-           units, so be careful to not step over that. ES3 on the same has 16. */
+           units, so be careful to not step over that. ES3 on the same has 16.
+           Not using the default (0) because this shader is quite specific.
+           Unit 6 is used by Shaders::Vector and Shaders::DistanceFieldVector. */
         enum: Int { TextureUnit = 7 };
 
         Int scalingUniform{0},