how to add new image sdecoder.

1. create a directory which name is your image decoder name and put source files
 to the directory. e.g. bmp.
const struct image_decoder image_decoder and IMGDEC_HEADER need to be defined in
 your souce file.
check imageviewer.h for the detail of struct image_decoder.

2. add the directory name to SUBDIR in this directory.
if your decoder is supported by particular targets, surround it with #if directive.
e.g. if the decoder supports color LCD targets only, 
#if HAVE_LCD_COLOR
bmp
#endif

3. append appropriate entry to enum image_type in image_decoder.h, decoder_names
 and ext_list in image_decoder.c

4. add entry to the apps/plugins/viewers.config. e.g. bmp,viewer/imageviewer
if your decoder is supported by particular targets, surround it with #if directive.
e.g. if the decoder supports color LCD targets only,
#if HAVE_LCD_COLOR
bmp,viewer/imageviewer
#endif

notes:
if you need to use greylib functions to draw image, add the functions to
 struct imgdec_api just like gray_bitmap_part.