Replace FreeImage dependency with stb

This removes FreeImage dependency. This library is full of security
vulnerabilities [1][2] and distributions such as NixOS and Arch no
longer ship it, making it difficult to build Gazebo there. It's not an
option to replace FreeImage with its fork called FreeImageRe [3],
which fixes some of the vulnerabilities, because many of them still
seem to be present. Therefore, we replace FreeImage with the stb
project [4], which seems to have better security track record [5]. We
vendor the header files from the project from its commit
f1c79c0 ("Merge pull request 1851 from jeffrbig2/master", 2025-10-25).

With this implementation all tests pass, but it is quite likely that
some things will behave differently. Likely the support of 16-bit or
HDR (32-bit float) images will behave differently. For example, PNG
saving is supported only for 8-bit images.

[1]: https://security-tracker.debian.org/tracker/source-package/freeimage
[2]: https://www.cvedetails.com/vendor/15676/
[3]: https://github.com/agruzdev/FreeImageRe
[4]: https://github.com/nothings/stb
[5]: https://www.cvedetails.com/vendor/19263/Stb-Project.html

Closes #388

Signed-off-by: Michal Sojka <michal.sojka@cvut.cz>

Author: wentasah

CMakeLists.txt

@@ -58,12 +58,6 @@ if(NOT WIN32)
   gz_find_package(UUID REQUIRED PRETTY uuid)
 endif()
 
-#------------------------------------
-# Find Freeimage
-gz_find_package(FreeImage VERSION 3.9
-  REQUIRED_BY graphics
-  PRIVATE_FOR graphics)
-
 #------------------------------------
 # Find GDAL
 gz_find_package(GDAL VERSION 3.0

graphics/src/CMakeLists.txt

@@ -8,7 +8,6 @@ target_link_libraries(${graphics_target}
     gz-utils::gz-utils
   PRIVATE
     ${GzAssimp_LIBRARIES}
-    FreeImage::FreeImage
     TINYXML2::TINYXML2
 )