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A general description is here.
Package tcltk now sends strings to Tcl in UTF-8: this means that strings with a marked UTF-8 encoding are supported in non-UTF-8 locales.
The graphics engine now supports rendering of raster (bitmap) images, though not all graphics devices can provide (full) support. Graphics device packages, e.g., RSvgDevice, will need to be reinstalled.
There is also support in the graphics engine for capturing raster images from graphics devices (again not supported on all graphics devices).
Package tcltk's demo(tkdensity) was broken in 2.9.0 when demo() was changed to set par(ask = TRUE).