[sdf] Fixed corner checks and improved performance.

* src/sdf/ftsdf.c (sdf_generate_bounding_box): Always check for
  corner if two distance (for different curves) are very close.

* src/sdf/ftsdf.c (sdf_conic_to): Added check to figure out if
  the conic can be treated as a line (which happens if the control
  point coincide with any end-point).
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