Icon paint safety and maintenance

For safety:
1. Whenever we get colors and alpha via cairo_pattern_get_rgba() we must check for a valid result,
   if not the case we should always not process further from here.
   Had been done in some cases already but we should do that always.
2. We check for a valid malloc()

Maintenance;
1. cairo expects double as parameters so avoid runtime conversions (except one case where we use floats
   presented via `data`
2. constify where possible
3. some minor code reduction