Added: in file dialog, multiple file selection.
    Added: Draw mode selector (in left toolbar):
        Raw mode:
            a Gerber image is drawn on screen without buffering.
            Artifacts happen if there are negative items drawn, if more than one  Gerber file is shown.
        Stacked mode:
            each Geber image is drawn in a buffer and after drawn on screen
            No artifact with negative items.
            Each Gerber image covers previous images.
        OR mode (transparency mode):
            each Geber image is drawn in a buffer and after drawn on screen
            No artifact with negative items.
            Each Gerber image is "ORed" with previous images, like in Pcbnew.
    Try to optimize Draw function in buffered modes.
        (Useful for PC that have problems with "blit" graphic function)
    Fix minor issues.

Pcbnew: For zone filling algo, change the default polygon library from Kbool to Boost::polygon (USE_BOOST_POLYGON_LIBRARY default is ON). (need to rebuild makefile)
Gerbview: added decimal format for coordinates, sometimes found in Gerber files.

Gerbview: Added support for gerber command   SR (Step and Repeat) and multiple MOIN and MOMM in file

Gerbview: added  support forSF  (scale factor), IO (Image Offset), OF (Offset) and MI (Mirror) commands.

Fixed a bug about Aperture Definition having parameters for an aperture macro. See Changlog for not yet supported RX274X commands. Having still problems with arcs in some gereber files.

Some cleanup and fixes in gerbview. Cleanup gr_basic.* and added in gr_basic.* a minor modification coming from "EEschema selection.patch".