speed testing libcairo

* in Cairo, push/pop group is added (code actually copied from Roger Henrikstad's email discussion)
* test directory established
* runtests.jl
* shape_functions.jl
   ddotsX (paint dots as discs - no ring/border)
   ddots1: build the complete path and then fill
   ddots2: fill single dots
   ddots3: fill single dots as 0-length lines with round caps
   ddots4: build group (with push/pop_group) and translate paint
   ddots5: render to a pattern and translate, paint.
   rdotsX (paint dots with a ring border)
   1...5, same as above
   linesX (line painting)
   lines0: random x,y
   lines1: x sorted, something like a lineplot, build the complete path, then stroke
   lines2: x sorted, independent stroke of all segments
   lines3: x sorted, build a path of 100 lines, then stroke
   lines4: x sorted, build a path of 1000 lines, then stroke

* test_speed.jl, script to run a bunch of tests on an ARGB image surface (size 512x512), different paint widths (dot size or line width), different shapes, different number of elements. collects the timing into an array (test_data).