Add support for images with 4 channels

[YavorGIvanov]

Currently the code in load_image_from_file(..) function is naively written to only support 3 channels and return error if this is not the case. This is very limiting as a lot of images have additional alpha channel in them and we are not able to test SAM with them due to that.

sam.cpp/examples/main.cpp

Line 18 in 39aa3eb

static bool load_image_from_file(const std::string & fname, sam_image_u8 & img) {

[bhavuk2002]

can you assign me this? i will work on this.