Add RmlUi to build system

Not all is ready and there will be more work needed on build system, but
the basic integration of RmlUi Core is done.

- I've not created and linked any of the provided GL3 or SDL backends,
  that needs separate work
- Lua bindings are blocked on mikke89/RmlUi#302
  we need to find some workaround for that
- Tracy integration is blocked on mikke89/RmlUi#516

.gitmodules

@@ -30,3 +30,6 @@
 [submodule "rts/lib/cereal"]
 	path = rts/lib/cereal
 	url = https://github.com/USCiLab/cereal.git
+[submodule "rts/lib/RmlUi"]
+	path = rts/lib/RmlUi
+	url = https://github.com/mikke89/RmlUi.git

CMakeLists.txt

@@ -70,6 +70,9 @@ else()
 	set(JAVA_COMPILE_FLAG_CONDITIONAL "-g:lines,source")
 endif()
 
+# By default, the libraries that don't explicily specify SHARED/STATIC are build statically.
+# See https://cmake.org/cmake/help/latest/variable/BUILD_SHARED_LIBS.html
+option(BUILD_SHARED_LIBS "Build shared (dynamic) libraries" OFF)
 
 # Tell windows to pass arguments around in @response files
 # (32k arg limit workaround)

rts/System/SpringApp.cpp

@@ -6,6 +6,9 @@
 #include <iostream>
 #include <chrono>
 
+// Include before other headers that define a bunch of macros that break stuff.
+#include <RmlUi/Core.h>
+
 #include <SDL.h>
 #include <System/GflagsExt.h>
 
@@ -217,6 +220,17 @@ SpringApp::~SpringApp()
 	spring_clock::PopTickRate();
 }
 
+class DummyRmlUiSpringSystemInterface : public Rml::SystemInterface {
+public:
+	double GetElapsedTime() override {
+		return 0.0;
+	}
+
+private:
+	std::chrono::steady_clock::time_point start;
+};
+
+static DummyRmlUiSpringSystemInterface dummyRmlUiSpringSystemInterface;
 
 /**
  * @brief Initializes the SpringApp instance
@@ -242,6 +256,9 @@ bool SpringApp::Init()
 	Watchdog::Install();
 	Watchdog::RegisterThread(WDT_MAIN, true);
 
+	// A dummy call to basic RmlUi function to verify that all is linking correctly.
+	Rml::SetSystemInterface(&dummyRmlUiSpringSystemInterface);
+
 	// Create Window
 	if (!InitWindow(("Spring " + SpringVersion::GetSync()).c_str())) {
 		SDL_Quit();

rts/builds/headless/CMakeLists.txt

@@ -27,6 +27,7 @@ list(APPEND engineHeadlessLibraries ${engineCommonLibraries})
 list(APPEND engineHeadlessLibraries no-sound)
 list(APPEND engineHeadlessLibraries engineSim)
 list(APPEND engineHeadlessLibraries pr-downloader)
+list(APPEND engineHeadlessLibraries RmlCore)
 
 include_directories(${ENGINE_SRC_ROOT_DIR}/lib/assimp/include)
 include_directories(${ENGINE_SRC_ROOT_DIR}/lib/asio/include)

rts/builds/legacy/CMakeLists.txt

@@ -36,6 +36,7 @@ if    (FONTCONFIG_FOUND)
 	endif (PREFER_STATIC_LIBS)
 endif (FONTCONFIG_FOUND)
 
+# !!! If you change freetype resolution here, change it also in rts/lib
 find_freetype_hack() # hack to find different named freetype.dll
 find_package_static(Freetype REQUIRED)
 foreach(f ${FREETYPE_INCLUDE_DIRS})
@@ -64,6 +65,7 @@ list(APPEND engineLibraries ${engineCommonLibraries})
 list(APPEND engineLibraries ${SPRING_SIM_LIBRARIES})
 list(APPEND engineLibraries engineSim)
 list(APPEND engineLibraries pr-downloader)
+list(APPEND engineLibraries RmlCore)
 
 ### Assemble external incude dirs
 list(APPEND engineIncludes ${OPENAL_INCLUDE_DIR})

rts/lib/CMakeLists.txt

@@ -85,3 +85,20 @@ option(TRACY_ON_DEMAND "Enable tracy profiling" ON)
 option(TRACY_PROFILE_MEMORY "Profile memory allocations" OFF)
 
 add_subdirectory(tracy)
+
+# TODO(p2004): Enabling tracy for RmlUi is at the momemnt broken and waits for
+#              resolution upstream https://github.com/mikke89/RmlUi/issues/516.
+# if (TRACY_ENABLE)
+# 	set(ENABLE_TRACY_PROFILING ON CACHE BOOL "Enable RmlUi tracy profiling")
+# 	# Place the include directories for tracy in TRACY_INCLUDE_DIR as that's
+# 	# what RmlUi expects to be set in their FindTracy.cmake
+# 	get_target_property(TRACY_INCLUDE_DIR TracyClient INCLUDE_DIRECTORIES)
+# endif()
+
+# We resolve freetype here to make sure RmlUi find_package picks the same
+# correct version as picked by the engine.
+# !!! If you change freetype resolution here, change it also in rts/builds/legacy
+find_freetype_hack() # hack to find different named freetype.dll
+find_package_static(Freetype REQUIRED)
+
+add_subdirectory(RmlUi)