obs-ffmpeg: Add ability to debug ffmpeg-mux subprocess

Adds a cmake variable (DEBUG_FFMPEG_MUX) which enables FFmpeg debug
output in the ffmpeg-mux subprocess, and if on Windows, shows the
console window of the ffmpeg-mux subprocess so the current output can be
seen.

plugins/obs-ffmpeg/ffmpeg-mux/CMakeLists.txt

@@ -1,5 +1,9 @@
 project(obs-ffmpeg-mux)
 
+if(DEBUG_FFMPEG_MUX)
+	add_definitions(-DDEBUG_FFMPEG)
+endif()
+
 find_package(FFmpeg REQUIRED
 	COMPONENTS avcodec avutil avformat)
 include_directories(${FFMPEG_INCLUDE_DIRS})