Vertical section missing value rendering bug fix

The vertical section has a bug, where missing values in the grid are still rendered when using the filled contours render option. This is due to an issue between the Vertex and Fragment shaders, where the missing value is lost and replaced with NaN values. This MR tries to fix that, by checking in the vertex shader, whether the value is a missing value, and setting the vertex flag to ignore it to -100, so that the fragment shader discards this fragment.