Add: [Emscripten] use "relative mouse mode" with SDL2

This mode doesn't wrap the mouse constantly, but requests SDL
to lock the mouse pointer. This is needed, as with Emscripten
you are not allowed to change the mouse poisition (only to lock
it into place).

src/gfx.cpp

@@ -1784,6 +1784,30 @@ void SetAnimatedMouseCursor(const AnimCursor *table)
 	SwitchAnimatedCursor();
 }
 
+/**
+ * Update cursor position on mouse movement for relative modes.
+ * @param delta_x How much change in the X position.
+ * @param delta_y How much change in the Y position.
+ */
+void CursorVars::UpdateCursorPositionRelative(int delta_x, int delta_y)
+{
+	if (this->fix_at) {
+		this->delta.x = delta_x;
+		this->delta.y = delta_y;
+	} else {
+		int last_position_x = this->pos.x;
+		int last_position_y = this->pos.y;
+
+		this->pos.x = Clamp(this->pos.x + delta_x, 0, _cur_resolution.width - 1);
+		this->pos.y = Clamp(this->pos.y + delta_y, 0, _cur_resolution.height - 1);
+
+		this->delta.x = last_position_x - this->pos.x;
+		this->delta.y = last_position_y - this->pos.y;
+
+		this->dirty = true;
+	}
+}
+
 /**
  * Update cursor position on mouse movement.
  * @param x New X position.

src/gfx_type.h

@@ -143,6 +143,7 @@ struct CursorVars {
 	/* Drag data */
 	bool vehchain;                ///< vehicle chain is dragged
 
+	void UpdateCursorPositionRelative(int delta_x, int delta_y);
 	bool UpdateCursorPosition(int x, int y, bool queued_warp);
 
 private:

src/video/sdl2_v.cpp

@@ -51,6 +51,11 @@ static volatile bool _draw_continue;
 static Palette _local_palette;
 static SDL_Palette *_sdl_palette;
 
+#ifdef __EMSCRIPTEN__
+/** Whether we just had a window-enter event. */
+static bool _cursor_new_in_window = false;
+#endif
+
 #define MAX_DIRTY_RECTS 100
 static SDL_Rect _dirty_rects[MAX_DIRTY_RECTS];
 static int _num_dirty_rects;
@@ -350,6 +355,9 @@ bool VideoDriver_SDL::CreateMainSurface(uint w, uint h, bool resize)
 bool VideoDriver_SDL::ClaimMousePointer()
 {
 	SDL_ShowCursor(0);
+#ifdef __EMSCRIPTEN__
+	SDL_SetRelativeMouseMode(SDL_TRUE);
+#endif
 	return true;
 }
 
@@ -509,9 +517,27 @@ int VideoDriver_SDL::PollEvent()
 
 	switch (ev.type) {
 		case SDL_MOUSEMOTION:
+#ifdef __EMSCRIPTEN__
+			if (_cursor_new_in_window) {
+				/* The cursor just moved into the window; this means we don't
+				* know the absolutely position yet to move relative from.
+				* Before this time, SDL didn't know it either, and this is
+				* why we postpone it till now. Update the absolute position
+				* for this once, and work relative after. */
+				_cursor.pos.x = ev.motion.x;
+				_cursor.pos.y = ev.motion.y;
+				_cursor.dirty = true;
+
+				_cursor_new_in_window = false;
+				SDL_SetRelativeMouseMode(SDL_TRUE);
+			} else {
+				_cursor.UpdateCursorPositionRelative(ev.motion.xrel, ev.motion.yrel);
+			}
+#else
 			if (_cursor.UpdateCursorPosition(ev.motion.x, ev.motion.y, true)) {
 				SDL_WarpMouseInWindow(_sdl_window, _cursor.pos.x, _cursor.pos.y);
 			}
+#endif
 			HandleMouseEvents();
 			break;
 
@@ -615,6 +641,12 @@ int VideoDriver_SDL::PollEvent()
 			} else if (ev.window.event == SDL_WINDOWEVENT_ENTER) {
 				// mouse entered the window, enable cursor
 				_cursor.in_window = true;
+#ifdef __EMSCRIPTEN__
+				/* Disable relative mouse mode for the first mouse motion,
+				 * so we can pick up the absolutely position again. */
+				_cursor_new_in_window = true;
+				SDL_SetRelativeMouseMode(SDL_FALSE);
+#endif
 			} else if (ev.window.event == SDL_WINDOWEVENT_LEAVE) {
 				// mouse left the window, undraw cursor
 				UndrawMouseCursor();
@@ -631,7 +663,8 @@ const char *VideoDriver_SDL::Start(const StringList &parm)
 	/* Explicitly disable hardware acceleration. Enabling this causes
 	 * UpdateWindowSurface() to update the window's texture instead of
 	 * its surface. */
-	SDL_SetHint(SDL_HINT_FRAMEBUFFER_ACCELERATION , "0");
+	SDL_SetHint(SDL_HINT_FRAMEBUFFER_ACCELERATION, "0");
+	SDL_SetHint(SDL_HINT_MOUSE_RELATIVE_MODE_WARP, "1");
 
 	/* Just on the offchance the audio subsystem started before the video system,
 	 * check whether any part of SDL has been initialised before getting here.