Gerbview: remove flicker when refresh screen. Pcbnew fix (partially) issue when printing in mirror mode ( version compiled  with USE_WX_ZOOM = ON specific). Minor enhancements.

Minor cosmetic enhancements.
commint forgoten files in boost lib.

    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.

    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.

Fix Gerbview issue (does not display gerber images) when Gerbview is compiled with USE_WX_ZOOM defined.

Pcbnew: fix crash after displaying "The text thickness is too large for the text size. It will be clamped" when editing pcb text properties (Linux specfic)

Other very minor changes.

soldermask. Nevertheless, it would be nice to have proper gerbers to
begin with, i.e., gerbers with no silkscreen in soldermaskless areas.
Of course, most of the problem can be taken care of by drawing the
footprints properly. However, sometimes the silkscreen of one
footprint ends up on the pads of another one (think components under a
so-dimm card, for example). The attached patch adds an option to
subtract the soldermask from the silkscreen when doing gerber output.

marco

Cosmetic enhancements. Eeschema: in libedit the Value Field is n more editable in the field editor dialog (due to recent changes in library code)

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.