brushlib: Add mypaint_brush_from_defaults()

A brush initializes itself with null mappings for all settings,
so one cannot actually draw with such a brush.
Also fixes comment on mypaint_brush_stroke_to()
brushlib: Fix minimal.c example

Need to call begin/end_atomic() around modifications, and to load
sensible default values for the brush. Also tweak the dtime to
a more sensible value.
brushlib: Fix a linker error when building with enable_gegl=true enable_gperftool=true

Also prettify output a bit.
brushlib: Add a benchmark/test for implementation details of TiledSurface

Useful to find exactly what is slow in previously identified hotspots.
brushlib: const correctness in mypaint-tiled-surface.c
brushlib: Avoid double/single precision conversions in render_dab_mask

render_dab_mask is about 10% faster as a result, and test-gegl-surface around 5%.

Jons-MacBook-Pro:mypaint jon$ ./brushlib/tests/test-details
render_dab_mask: 10255 ms

Jons-MacBook-Pro:mypaint jon$ ./brushlib/tests/test-details
render_dab_mask: 8846 ms
brushlib: Use float literals everywhere in mypaint-tiled-surface.c

Causes single/double precision conversions. Does not actually lead
to better performance when not in inner loops, but at least we are
consistent.
Merge branch 'mypaint-head'