Optimize Shadow drawing

- Related to #345
- for text with no shadow only the glyph run is drawn
- for text with 1 shadow this is set and the glyph run is drawn
- for text with more than 1 shadow an area is clipped, 100 pixel wider in all directions
  - then the text position is moved outside that area and the shadow gets an additional offset in the other direction
  - this way only the shadow is drawn
  - except for the very last shadow, there the text position is moved back and the glyph run is drawn together with the last shadow
renamed define for showing parsing to DEBUG_LOG_METRICS
closes #346