Expose skip_frames as a build option

It can be initially configured by:

    meson builddir -Dskip_frames=false

Or on an existing builddir by:

    mesonconf builddir -Dskip_frames=false

app/meson.build

@@ -74,7 +74,7 @@ conf.set('DEFAULT_BIT_RATE', '4000000')  # 4Mbps
 # whether the app should always display the most recent available frame, even
 # if the previous one has not been displayed
 # SKIP_FRAMES improves latency at the cost of framerate
-conf.set('SKIP_FRAMES', true)
+conf.set('SKIP_FRAMES', get_option('skip_frames'))
 
 configure_file(configuration: conf, output: 'config.h')