Uses the new structure agreed upon in #6157, but the thirdparty/ folder
does not behave following a logic similar to that of modules/ yet.
The png driver can't be moved to a module as discussed in #6157, as it's
required by core together with a few other ImageLoader implementations
(see drivers/register_driver_types.cpp:register_core_driver_types())
Dropped the possibility to disable PNG support, it's a core component
of Godot.
Warning: libpng 1.6 is strict about iCCp chunks being incorrect,
solution is to fix the profile. On *nix ImageMagicks "convert" works
on windows optiPNG.
