Add Image::diff.

Fixed inavalid memory access due to double free in FontBMP and FontSprite.
Added FontRendering unit-tests.

.github/workflows/eepp-macos-build-check.yml

@@ -16,7 +16,7 @@ jobs:
         git submodule update --init --recursive
     - name: Install dependencies
       run: |
-        brew install wget SDL2 premake
+        brew install wget SDL2 premake mesa
     - name: Build
       run: |
         premake5 --with-text-shaper --disable-static-build gmake
@@ -24,5 +24,9 @@ jobs:
     - name: Unit Tests
       run: |
         cd bin/unit_tests
+        export LIBGL_ALWAYS_SOFTWARE=1
+        export GALLIUM_DRIVER=llvmpipe
+        export SDL_VIDEODRIVER=cocoa
         ./eepp-unit_tests
-
+      env:
+        DYLD_LIBRARY_PATH: /opt/homebrew/lib  # Ensures Mesa's libGL is used