[appveyor] use stb from vcpkg when not testing integrated libs

appveyor.yml

@@ -67,7 +67,8 @@ install:
   - if [%COMPILER%]==[vs] if NOT [%USE_INTEGRATED_LIBS%]==[yes] git pull
   - if [%COMPILER%]==[vs] if NOT [%USE_INTEGRATED_LIBS%]==[yes] .\bootstrap-vcpkg.bat
   - if [%COMPILER%]==[vs] if NOT [%USE_INTEGRATED_LIBS%]==[yes] if [%USE_CUDA%]==[yes] vcpkg install cuda
-  - if [%COMPILER%]==[vs] if NOT [%USE_INTEGRATED_LIBS%]==[yes] vcpkg install opencv[ffmpeg] pthreads   ## opencv[ffmpeg,cuda] is too big to build, timing out (>1h). We use plain openCV also for CUDA builds (toolchain can manage this strange situation anyway)
+  - if [%COMPILER%]==[vs] if NOT [%USE_INTEGRATED_LIBS%]==[yes] vcpkg install stb pthreads
+  - if [%COMPILER%]==[vs] if NOT [%USE_INTEGRATED_LIBS%]==[yes] vcpkg install opencv[ffmpeg] ## opencv[ffmpeg,cuda] is too big to build, timing out (>1h). We use plain openCV also for CUDA builds (toolchain can manage this strange situation anyway)
   - cd %WORKSPACE%\darknet\
   - mkdir build_debug && cd build_debug
   - if [%COMPILER%]==[cygwin] %CYGSH% 'cmake .. -G "Unix Makefiles" -DCMAKE_BUILD_TYPE="Debug"'