Allow passing nullptr to IndexedMesh::setIndexBuffer().

Similar functionality should be in Mesh itself for unbinding vertex
buffers.

src/IndexedMesh.cpp

@@ -42,7 +42,7 @@ void IndexedMesh::draw() {
 }
 
 void IndexedMesh::bind() {
-    CORRADE_ASSERT(_indexCount, "IndexedMesh: the mesh has zero index count!", );
+    CORRADE_ASSERT(!_indexCount || _indexBuffer, "IndexedMesh: index buffer must be added if index count is non-zero", );
 
     Mesh::bind();
     (this->*bindIndexedImplementation)();
@@ -66,11 +66,13 @@ void IndexedMesh::bindIndexBufferImplementationDefault() {}
 
 void IndexedMesh::bindIndexBufferImplementationVAO() {
     bindVAO(vao);
-    _indexBuffer->bind(Buffer::Target::ElementArray);
+    if(_indexBuffer) _indexBuffer->bind(Buffer::Target::ElementArray);
+    else Buffer::unbind(Buffer::Target::ElementArray);
 }
 
 void IndexedMesh::bindIndexedImplementationDefault() {
-    _indexBuffer->bind(Buffer::Target::ElementArray);
+    if(_indexBuffer) _indexBuffer->bind(Buffer::Target::ElementArray);
+    else Buffer::unbind(Buffer::Target::ElementArray);
 }
 
 void IndexedMesh::bindIndexedImplementationVAO() {}

src/IndexedMesh.h

@@ -68,6 +68,9 @@ class MAGNUM_EXPORT IndexedMesh: public Mesh {
         /**
          * @brief Set index buffer
          *
+         * By default there is no index buffer. Parameter @p buffer can be
+         * `nullptr`, in that case current index buffer is unbound from the
+         * mesh.
          * @see MeshTools::compressIndices(), @fn_gl{BindVertexArray},
          *      @fn_gl{BindBuffer} (if @extension{APPLE,vertex_array_object}
          *      is available)

src/Mesh.h

@@ -112,6 +112,8 @@ more information.
 
 @todo Support for indirect draw buffer (OpenGL 4.0, @extension{ARB,draw_indirect})
 @todo Redo in a way that allows glMultiDrawArrays, glDrawArraysInstanced etc.
+@todo Allow unbinding all vertex buffers with some function (not as side effect),
+    similarly to unbinding index buffer in IndexedMesh
  */
 class MAGNUM_EXPORT Mesh {
     friend class IndexedMesh;