fix build error

src/mol-gl/compute/marching-cubes/isosurface.ts

@@ -62,7 +62,7 @@ function getIsosurfaceRenderable(ctx: WebGLContext, activeVoxelsPyramid: Texture
     };
 
     const schema = { ...IsosurfaceSchema };
-    const shaderCode = ShaderCode('isosurface', quad_vert, isosurface_frag, { drawBuffers: true });
+    const shaderCode = ShaderCode('isosurface', quad_vert, isosurface_frag, { drawBuffers: 'required' });
     const renderItem = createComputeRenderItem(ctx, 'triangles', shaderCode, schema, values);
 
     return createComputeRenderable(renderItem, values);