* video/reader/png.c (grub_png_data): New member is_16bit and
image_data.
(grub_png_decode_image_header): Detect 16 bit png image.
(grub_png_convert_image): New function to convert 16 bit image to 8 bit.
(grub_png_decode_png): Call grub_png_convert_image for 16 bit image.
(grub_video_reader_png): Release memory occupied by image_data.
* fs/ntfs.c (find_attr): Handle non-resident attribute list larger than
4096 bytes.
(grub_nfs_mount): Skip the test for sector per cluster.