lv_image_flags_t enum

For RGB map of the image data, mark if it's pre-multiplied with alpha. For indexed image, this bit indicated palette data is pre-multiplied with alpha.

The image data is compressed, so decoder needs to decode image firstly. If this flag is set, the whole image will be decompressed upon decode, and `get_area_cb` won't be necessary.

The image is allocated from heap, thus should be freed after use.

If the image data is malloced and can be processed in place. In image decoder post processing, this flag means we modify it in-place.

Flags reserved for user, lvgl won't use these bits.