Use Pango for font layout
Correctly scale offset for BoxAlignedText
Actually use PangoLayout for rendering text

not just for querying extent.  Handle Local FontSize by scaling before
layout.  This looks better, but it's possible we should implement both
interpretations.
restore semantics of Local FontSize
Use liftIO instead of unsafePerformIO
Handle non-centered Text

Pango's origin is at upper left, so for (BoxAlignedText 0 1) we want an
offset r2 (0,0).  Cairo's origin is at baseline, so handling BaselineText
used to be trivial.
Revised semantics of Local FontSize

layout text at final output size, apply tn, the Transformation
normalized to avgScale 1

Revert "restore semantics of Local FontSize"

This reverts commit d1a70cb6de9694debe51bbb1fca9a79cf0102093.

Conflicts:
	src/Diagrams/Backend/Cairo/Internal.hs
Merge pull request #49 from diagrams/pango

Pango, closes #19