misc: image: refactor, fix ImageWrite, and add new test for it

Refactor code to always call the encoder at the end and separate the filtering case.

Fix a crash (double-free) when the image handler needs to resize the picture before encoding it.

==150429==ERROR: AddressSanitizer: heap-use-after-free on address 0x614000002bb8 at pc 0x7f4ce1a8a092 bp 0x7fff205294c0 sp 0x7fff205294b0
WRITE of size 8 at 0x614000002bb8 thread T0
    #0 0x7f4ce1a8a091 in vlc_atomic_rc_dec ../../include/vlc_atomic.h:66
    #1 0x7f4ce1a8a091 in picture_Release ../../include/vlc_picture.h:372
    #2 0x7f4ce1a8a091 in ImageWrite ../../src/misc/image.c:454
    #3 0x564f4f6a00f7 in OpenIntf ../../test/src/misc/image.c:52
    #4 0x7f4ce177bfee in generic_start ../../src/modules/modules.c:275
    #5 0x7f4ce177db75 in vlc_module_load ../../src/modules/modules.c:243
    #6 0x7f4ce177df33 in module_need ../../src/modules/modules.c:286
    #7 0x7f4ce179cbdd in intf_Create ../../src/interface/interface.c:172
    #8 0x7f4ce179d86a in libvlc_InternalAddIntf ../../src/interface/interface.c:267
    #9 0x7f4ce2350b22 in libvlc_add_intf ../../lib/playlist.c:41
    #10 0x564f4f69f53c in main ../../test/src/misc/image.c:122