Image-based term layers (no CAShapeLayer).
EXTTermLayer is now a protocol with two conforming classes: EXTShapeTermLayer, derived from CAShapeLayer, and EXTImageTermLayer, derived from CALayer. EXTChartView.vectorChart tells the chart view if it should use shape- or image-based layers. Vector charts are better suited to PDF export.
Abstracts common data & behaviour from EXT{Image,Shape}TermLayer into EXTTermLayer & EXTTermLayerSurrogate. Renames selected to selectedObject because of conflict with -[CALayer(LayerExtra) setSelected:] in Quartz/ImageKit.
Export to PDF now uses a second EXTChartView instance configured with vectorChart = true.
Fixes headers.
Fixes a differential selection bug where multiple differentials could be rendered as selected. Thank you, string-based programming.
Merge branch 'imgterm'