Add libpng-apng as a new port

[alberthdev]

I've added libpng-apng as a possible new port for vcpkg. It's pretty much based off the existing libpng port, except it now downloads a patch to apply before building.

The only issue with this particular port is that CMake doesn't seem to support extracting *.gz files for some reason, which is the format that the patch is in.

Error follows:

CMake Error: Problem with archive_read_open_file(): Unrecognized archive format
CMake Error: Problem extracting tar: C:/Users/SomeUser/Documents/GitHub/vcpkg/downloads/libpng-1.6.34-apng.patch.gz

I'm not 100% sure if I'm calling this incorrectly or not. I don't see any CMake documentation indicating they support GZip compression directly - though they do say they support .tar.gz, which is interesting...

Any idea if this is just a mistaken call, or if this is indeed a missing implementation from CMake? (And if so, do we want to request such a feature from them, or alternatively just download and run some sort of g(un)zip.exe?

[msftclas]

All CLA requirements met.

[ras0219-msft]

Thanks for the PR!

Yeah, I don't see any ability to un-gzip directly (only .tar.gz). I think the best way to handle this will be to download the x86 executable for 7zip (via vcpkg_find_acquire_program(7Z)) and use that.

[alberthdev]

Just updated my PR to do exactly that, plus some additional fixes as well to the portfile.

[ras0219-msft]

Thanks!