Fixed build.

src/renderer_mtl.mm

@@ -1475,9 +1475,9 @@ namespace bgfx { namespace mtl
 			m_uniformBufferFragmentOffset += fragmentUniformBufferSize;
 			m_uniformBufferVertexOffset = m_uniformBufferFragmentOffset;
 
-			VertexBufferMtl& vb = m_vertexBuffers[_clearQuad.m_vb->handle.idx];
+			const VertexBufferMtl& vb = m_vertexBuffers[_clearQuad.m_vb->handle.idx];
 			const VertexDecl& vertexDecl = m_vertexDecls[_clearQuad.m_vb->decl.idx];
--			const uint32_t stride = vertexDecl.m_stride;
+			const uint32_t stride = vertexDecl.m_stride;
 			const uint32_t offset = 0;
 
 			{