Add support for multiple screens (#876)

In the config file, in the general section, add a line with "screen" and a value of 1.
<section name="general">
        <var name="screen" value="1" />
Fixes #863.

engine/src/configuration/configuration.cpp

@@ -42,6 +42,8 @@ Configuration::Configuration() {
 * behavior/range bounding is complicated/necessary-for-stability abd additional validation code is required for checking/adjusting user inputs
 */
 void Configuration::OverrideDefaultsWithUserConfiguration() {
+    general_config.screen = GetGameConfig().GetInt16("general.screen", general_config.screen);
+
     general_config.pitch = GetGameConfig().GetFloat("general.pitch", general_config.pitch);
     general_config.yaw = GetGameConfig().GetFloat("general.yaw", general_config.yaw);
     general_config.roll = GetGameConfig().GetFloat("general.roll", general_config.roll);

engine/src/configuration/configuration.h

@@ -36,6 +36,9 @@ namespace vega_config {
 struct GeneralConfig {
     GeneralConfig() = default;
 
+    int screen{0};  // TODO: move to a dedicated section along with other such paramters.
+                    // e.g. screen width and height, resolution, color, etc.
+
     float pitch{0.0F};
     float yaw{0.0F};
     float roll{0.0F};

engine/src/gldrv/winsys.cpp

@@ -50,6 +50,7 @@
 #include "vs_logging.h"
 #include "options.h"
 #include "vs_exit.h"
+#include "configuration/configuration.h"
 
 #include "SDL2/SDL_video.h"
 
@@ -253,8 +254,17 @@ static bool setup_sdl_video_mode(int *argc, char **argv) {
     width = g_game.x_resolution;
     height = g_game.y_resolution;
 
-
-    SDL_Window *window = SDL_CreateWindow("Vegastrike", 0, 0, width, height, video_flags);
+    const int screen_number = configuration()->general_config.screen;
+    SDL_Window *window = nullptr;
+    if(screen_number == 0) {
+        window = SDL_CreateWindow("Vegastrike", 0, 0, width, height, video_flags);
+    } else {
+        window = SDL_CreateWindow("Vegastrike", 
+                                SDL_WINDOWPOS_UNDEFINED_DISPLAY(screen_number),
+                                SDL_WINDOWPOS_UNDEFINED_DISPLAY(screen_number), 
+                                0, 0, video_flags);
+    }
+
 
     if(!window) {
         std::cerr << "No window\n" << std::flush;