Encode missing values in heatmaps

[mstreit]

Coming from https://github.com/Caleydo/taggle/issues/316

In the rendered version we want to indicate missing values by a dash and via a provided color.
The texture-based heat map version should only use the color to indicate missing values (because the cells are usually much smaller).

The missing value color should set as a parameter.

[thinkh]

So far it only works for the heatmap canvas renderer. @sgratzl will fix the DOM renderer soon.

[sgratzl]

without any changes I get for the dom renderer: