Audio: Add MIDI format playback support

This adds a new dependency with libfuildsynth. MIDI support
is built by default, but can be disabled if not desired.

All RTP songs should work well, but there are known problems
with other files (see README). Also, the pitch shift implementation
is somewhat poor and doesn't match RMXP (at least subjectively).

A soundfont is not included and must be provided by
the user themself.

CMakeLists.txt

@@ -4,6 +4,7 @@ Project(mkxp)
 ## Setup options ##
 
 option(RGSS2 "Enable RGSS2" OFF)
+option(MIDI "Enable midi support" ON)
 option(FORCE32 "Force 32bit compile on 64bit OS" OFF)
 set(BINDING "MRI" CACHE STRING "The Binding Type (MRI, MRUBY, NULL)")
 set(EXTERNAL_LIB_PATH "" CACHE PATH "External precompiled lib prefix")
@@ -216,6 +217,19 @@ if (RGSS2)
 	)
 endif()
 
+if (MIDI)
+	pkg_check_modules(MIDI REQUIRED fluidsynth)
+	list(APPEND DEFINES
+		MIDI
+	)
+	list(APPEND MAIN_HEADERS
+		src/sharedmidistate.h
+	)
+	list(APPEND MAIN_SOURCE
+		src/midisource.cpp
+	)
+endif()
+
 ## Process Embeddeds ##
 
 find_program(XXD_EXE xxd
@@ -374,6 +388,7 @@ target_include_directories(${PROJECT_NAME} PRIVATE
 	${Boost_INCLUDE_DIR}
 	${MRI_INCLUDE_DIRS}
 	${RGSS2_INCLUDE_DIRS}
+	${MIDI_INCLUDE_DIRS}
 	${OPENAL_INCLUDE_DIR}
 )
 
@@ -388,6 +403,7 @@ target_link_libraries(${PROJECT_NAME}
 	${Boost_LIBRARIES}
 	${MRI_LIBRARIES}
 	${RGSS2_LIBRARIES}
+	${MIDI_LIBRARIES}
 	${OPENAL_LIBRARY}
 	${ZLIB_LIBRARY}