Remove vendored copies of 3rdparty bimg dependencies.
No more installed 3rdparty static libraries from bimg.
Add `BGFX_CMAKE_USER_SCRIPT` as entry point to change functionality of
bgfx.cmake config without the need for patches.
Insert miniz, tinyexr and libsquish from vcpkg repo into config.
Remove vcpkg's nvtt because bgfx's is completely different.
Co-authored-by: Jonliu1993 <13720414433@163.com>
