Shaders: don't exceed limits for no reason.

Shader::maxTextureImageUnits(Shader::Type::Fragment) is 16 on Ivy Bridge
and Mesa.

src/Magnum/Shaders/AbstractVector.h

@@ -62,7 +62,7 @@ template<UnsignedInt dimensions> class AbstractVector: public AbstractShaderProg
              * Vector texture binding unit
              * @deprecated Use @ref Magnum::Shaders::AbstractVector::setVectorTexture() "setVectorTexture()" instead.
              */
-            VectorTextureLayer = 16
+            VectorTextureLayer = 15
         };
         #endif
 
@@ -77,7 +77,7 @@ template<UnsignedInt dimensions> class AbstractVector: public AbstractShaderProg
         ~AbstractVector() = default;
 
         #ifndef MAGNUM_BUILD_DEPRECATED
-        enum: Int { VectorTextureLayer = 16 };
+        enum: Int { VectorTextureLayer = 15 };
         #endif
 };
 

src/Magnum/Shaders/DistanceFieldVector.frag

@@ -49,7 +49,7 @@ uniform lowp float smoothness;
 #endif
 
 #ifdef EXPLICIT_TEXTURE_LAYER
-layout(binding = 16) uniform sampler2D vectorTexture;
+layout(binding = 15) uniform sampler2D vectorTexture;
 #else
 uniform lowp sampler2D vectorTexture;
 #endif

src/Magnum/Shaders/Vector.frag

@@ -38,7 +38,7 @@ uniform lowp vec4 color;
 #endif
 
 #ifdef EXPLICIT_TEXTURE_LAYER
-layout(binding = 16) uniform sampler2D vectorTexture;
+layout(binding = 15) uniform sampler2D vectorTexture;
 #else
 uniform lowp sampler2D vectorTexture;
 #endif