extlib: update libpng to v1.6.50

This solves an issue with CMake 4.0.0, which doesn't support configs
for versions earlier than 3.5.

This unfortunately also requires messing with the way we include zlib,
because the static libpng target isn't really compatible with the way
zlib is built.

Author: iscgar

CMakeLists.txt

@@ -243,9 +243,22 @@ if(ENABLE_GUI OR ENABLE_CLI)
         set(FORCE_VENDORED_PNG      ON)
         set(FORCE_VENDORED_Freetype ON)
 
-        find_vendored_package(ZLIB zlib
-            ZLIB_LIBRARY            zlibstatic
-            ZLIB_INCLUDE_DIR        ${CMAKE_SOURCE_DIR}/extlib/zlib)
+        # HACK: This is essentially what `find_vendored_package()` does, except we also
+        # define the ZLIB::ZLIB target to be an alias for zlibstatic before calling
+        # `find_package()` in order to avoid having it defined as an imported target
+        # by the built-in FindZLIB.cmake module, which breaks libpng's png_static
+        # target that relies on ZLIB::ZLIB instead of on zlibstatic.
+        # Once zlib releases its develop branch post 1.3.1, the built-in module will not
+        # generate an imported target since we'll have proper targets, but libpng would
+        # still need to be updated to use ZLIB::ZLIBSTATIC instead of ZLIB::ZLIB, so this
+        # ugly hack will have to stay until both libraries release fixed versions.
+        add_vendored_subdirectory(extlib/zlib)
+        set(ZLIB_LIBRARY      zlibstatic CACHE INTERNAL "")
+        set(ZLIB_INCLUDE_DIR  ${CMAKE_SOURCE_DIR}/extlib/zlib CACHE INTERNAL "")
+        add_library(ZLIB::ZLIB ALIAS ${ZLIB_LIBRARY})
+        find_package(ZLIB REQUIRED)
+        set(ZLIB_FOUND        ${ZLIB_FOUND} CACHE INTERNAL "")
+
         list(APPEND ZLIB_INCLUDE_DIR ${CMAKE_BINARY_DIR}/extlib/zlib)
 
         # There's an issue with the zlib configuration, which doesn't provide a way to
@@ -266,10 +279,9 @@ if(ENABLE_GUI OR ENABLE_CLI)
         find_vendored_package(PNG libpng
             SKIP_INSTALL_ALL        ON
             PNG_LIBRARY             png_static
-            PNG_ARM_NEON            "off"
             PNG_SHARED              OFF
             PNG_STATIC              ON
-            PNG_EXECUTABLES         OFF
+            PNG_TOOLS               OFF
             PNG_TESTS               OFF
             PNG_FRAMEWORK           OFF
             PNG_PNG_INCLUDE_DIR     ${CMAKE_SOURCE_DIR}/extlib/libpng)