lv_image_flags_t enum

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.

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.