Fix filter area adjustment

Fixes #27 (closed)

This patch attempts to replace the single blur based area adjustment with a generic all-primatives area adjustment.

Each primitive can modify an OptRect to indicate what space it would take up for automatic detection of the filter's area.

• Gaussian Blur adjustments still work from object's panel
• Blur adjustments from filter's panel now work
• Offset adjustments now work (i.e. drop shadow)
• Error in filter rendering with offset :-/