MeshTools: fix compile() with a non-trivial index offset.

Haha, what the heck, how did nobody hit this yet.

doc/changelog.dox

@@ -530,6 +530,8 @@ See also:
 -   Fixed an assertion when using @ref MeshTools::removeDuplicates() on an
     interleaved @ref Trade::MeshData that included padding at the beginning or
     end of each vertex
+-   Fixed @ref MeshTools::compile() to properly take into account index buffer
+    offsets
 -   Fixed @ref MeshTools::removeDuplicates() to not take into account random
     padding bytes and filtered-out attributes in interleaved source
     @ref Trade::MeshData. This was a particularly glaring issue when using

src/Magnum/MeshTools/Compile.cpp

@@ -154,7 +154,7 @@ GL::Mesh compileInternal(const Trade::MeshData& meshData, GL::Buffer&& indices,
     }
 
     if(meshData.isIndexed()) {
-        mesh.setIndexBuffer(std::move(indices), 0, meshData.indexType())
+        mesh.setIndexBuffer(std::move(indices), meshData.indexOffset(), meshData.indexType())
             .setCount(meshData.indexCount());
     } else mesh.setCount(meshData.vertexCount());
 

src/Magnum/MeshTools/Test/CompileGLTest.cpp

@@ -445,6 +445,7 @@ template<class T> void CompileGLTest::twoDimensions() {
                 Containers::arraySize(vertexData), sizeof(Vertex))});
 
     const UnsignedInt indexData[]{
+        66, 78, 23, /* offset */
         0, 1, 4, 0, 4, 3,
         1, 2, 5, 1, 5, 4,
         3, 4, 7, 3, 7, 6,
@@ -452,7 +453,7 @@ template<class T> void CompileGLTest::twoDimensions() {
     };
 
     Trade::MeshData meshData{MeshPrimitive::Triangles,
-        {}, indexData, Trade::MeshIndexData{indexData},
+        {}, indexData, Trade::MeshIndexData{Containers::arrayView(indexData).suffix(3)},
         {}, vertexData, std::move(attributeData)};
 
     /* Duplicate everything if data is non-indexed */
@@ -672,6 +673,7 @@ template<class T> void CompileGLTest::threeDimensions() {
                 Containers::arraySize(vertexData), sizeof(Vertex))});
 
     const UnsignedByte indexData[]{
+        66, 78, 23, /* offset */
         0, 1, 4, 0, 4, 3,
         1, 2, 5, 1, 5, 4,
         3, 4, 7, 3, 7, 6,
@@ -679,7 +681,7 @@ template<class T> void CompileGLTest::threeDimensions() {
     };
 
     Trade::MeshData meshData{MeshPrimitive::Triangles,
-        {}, indexData, Trade::MeshIndexData{indexData},
+        {}, indexData, Trade::MeshIndexData{Containers::arrayView(indexData).suffix(3)},
         {}, vertexData, std::move(attributeData)};
 
     /* Duplicate everything if data is non-indexed */