combine text and icon render types
add symbol-min-distance, symbol-allow-overlap and

... symbol-ignore-placement
add doc and default for text-max-angle
make icon-rotate-anchor a render property

This needs to be considered for placement.
add `symbol-placement` and `-rotation-alignment`
add text-upgright and icon-upright

to control whether the symbols are flipped when rotated
symbol-rotation-alignment, not icon-rotation-alignment
add icon-max-size and icon-size
properties for sdf icons
unmerge some properties, drop icon-sdf
add icon-allow-without-text and text-allow-without-icon
*-allow-without-* replaced with *-optional
Merge pull request #79 from mapbox/render-symbol-2

Symbol overhaul, update to v4