test: add -w option to wait before app exit for decode test

it shows jpeg file better
XXX: there is no redraw upon expose, it keeps code simple though sometimes image doesn't show well.
jpegdec: handle comment marker and continue scan after current filed

simple bypass COM (0xfe) field like APPn marker
a second scan starts after end of current field, not after current marker.
test: fix when there is embeded thumbnail in jpeg file
tests: fix build when --enable-tests-gles is not enabled