Shaders: fix MeshVisualizer multidraw vertex ID test colormap wraparound.

Forgot that gl_VertexID includes the base offset in the multidraw
scenarios, so we need to take all vertices into account, not just the
largest view. The wraparound would cause nasty output differences among
drivers.

src/Magnum/Shaders/Test/MeshVisualizerGLTest.cpp

@@ -3924,7 +3924,8 @@ void MeshVisualizerGLTest::renderMulti2D() {
     if(data.flags & MeshVisualizerGL2D::Flag::NoGeometryShader)
         for(Trade::MeshData* i: {&circleData, &squareData, &triangleData})
             *i = MeshTools::duplicate(*i);
-    GL::Mesh mesh = MeshTools::compile(MeshTools::concatenate({circleData, squareData, triangleData}));
+    Trade::MeshData concatenated = MeshTools::concatenate({circleData, squareData, triangleData});
+    GL::Mesh mesh = MeshTools::compile(concatenated);
     GL::MeshView circle{mesh};
     circle.setCount(data.flags & MeshVisualizerGL2D::Flag::NoGeometryShader ?
         circleData.vertexCount() : circleData.indexCount());
@@ -3952,7 +3953,9 @@ void MeshVisualizerGLTest::renderMulti2D() {
         .setColor(0xffffcc_rgbf)
         .setWireframeColor(0xcc0000_rgbf);
     if(data.flags & MeshVisualizerGL2D::Flag::VertexId)
-        materialData[0*data.uniformIncrement].setColorMapTransformation(0.5f/circleData.vertexCount(), 1.0f/circleData.vertexCount());
+        /* Here, gl_VertexID is taken *including* the base offset, which means
+           we have to count all vertices to avoid colormap wraparounds */
+        materialData[0*data.uniformIncrement].setColorMapTransformation(0.5f/concatenated.vertexCount(), 1.0f/concatenated.vertexCount());
     else if(data.flags & MeshVisualizerGL2D::Flag::ObjectId)
         /* There's at most 4 colors (one every 2 faces) per draw and 3 draws,
            so make it fit 12 colors */
@@ -3962,7 +3965,9 @@ void MeshVisualizerGLTest::renderMulti2D() {
         .setWireframeColor(0x0000cc_rgbf)
         .setWireframeWidth(2.5f);
     if(data.flags & MeshVisualizerGL2D::Flag::VertexId)
-        materialData[1*data.uniformIncrement].setColorMapTransformation(0.5f/triangleData.vertexCount(), 1.0f/triangleData.vertexCount());
+        /* Here, gl_VertexID is taken *including* the base offset, which means
+           we have to count all vertices to avoid colormap wraparounds */
+        materialData[1*data.uniformIncrement].setColorMapTransformation(0.5f/concatenated.vertexCount(), 1.0f/concatenated.vertexCount());
     else if(data.flags & MeshVisualizerGL2D::Flag::ObjectId)
         /* There's at most 4 colors (one every 2 faces) per draw and 3 draws,
            so make it fit 12 colors */
@@ -4154,7 +4159,8 @@ void MeshVisualizerGLTest::renderMulti3D() {
     if(data.flags & MeshVisualizerGL3D::Flag::NoGeometryShader)
         for(Trade::MeshData* i: {&sphereData, &planeData, &coneData})
             *i = MeshTools::duplicate(*i);
-    GL::Mesh mesh = MeshTools::compile(MeshTools::concatenate({sphereData, planeData, coneData}));
+    Trade::MeshData concatenated = MeshTools::concatenate({sphereData, planeData, coneData});
+    GL::Mesh mesh = MeshTools::compile(concatenated);
     GL::MeshView sphere{mesh};
     sphere.setCount(data.flags & MeshVisualizerGL3D::Flag::NoGeometryShader ?
         sphereData.vertexCount() : sphereData.indexCount());
@@ -4189,7 +4195,9 @@ void MeshVisualizerGLTest::renderMulti3D() {
         .setWireframeColor(0xcc0000_rgbf)
         .setLineLength(0.0f); /* no TBN */
     if(data.flags & MeshVisualizerGL3D::Flag::VertexId)
-        materialData[0*data.uniformIncrement].setColorMapTransformation(0.5f/sphereData.vertexCount(), 1.0f/sphereData.vertexCount());
+        /* Here, gl_VertexID is taken *including* the base offset, which means
+           we have to count all vertices to avoid colormap wraparounds */
+        materialData[0*data.uniformIncrement].setColorMapTransformation(0.5f/concatenated.vertexCount(), 1.0f/concatenated.vertexCount());
     else if(data.flags & MeshVisualizerGL3D::Flag::ObjectId)
         /* There's at most 10 colors (one every 2 faces) per draw and 3 draws,
            so make it fit 30 colors */
@@ -4200,7 +4208,9 @@ void MeshVisualizerGLTest::renderMulti3D() {
         .setLineLength(0.25f)
         .setWireframeWidth(2.5f);
     if(data.flags & MeshVisualizerGL3D::Flag::VertexId)
-        materialData[1*data.uniformIncrement].setColorMapTransformation(0.5f/coneData.vertexCount(), 1.0f/coneData.vertexCount());
+        /* Here, gl_VertexID is taken *including* the base offset, which means
+           we have to count all vertices to avoid colormap wraparounds */
+        materialData[1*data.uniformIncrement].setColorMapTransformation(0.5f/concatenated.vertexCount(), 1.0f/concatenated.vertexCount());
     else if(data.flags & MeshVisualizerGL3D::Flag::ObjectId)
         /* There's at most 10 colors (one every 2 faces) per draw and 3 draws,
            so make it fit 30 colors */