Fix scroll wheel mouse position

SDL_MouseWheelEvent does not provide the mouse location, so we used
SDL_GetMouseState() to retrieve it.

Unfortunately, SDL_GetMouseState() returns a position expressed in the
window coordinate system while the position filled in SDL events are
expressed in the renderer coordinate system. As a consequence, the
scroll was not applied at the right position on the device.

Therefore, convert the coordinate system.

See <https://stackoverflow.com/questions/49111054/how-to-get-mouse-position-on-mouse-wheel-event>.

app/src/inputmanager.c

@@ -4,10 +4,24 @@
 #include "lockutil.h"
 #include "log.h"
 
-static struct point get_mouse_point(void) {
+// Convert window coordinates (as provided by SDL_GetMouseState() to renderer coordinates (as provided in SDL mouse events)
+//
+// See my question:
+// <https://stackoverflow.com/questions/49111054/how-to-get-mouse-position-on-mouse-wheel-event>
+static void convert_to_renderer_coordinates(SDL_Renderer *renderer, int *x, int *y) {
+    SDL_Rect viewport;
+    float scale_x, scale_y;
+    SDL_RenderGetViewport(renderer, &viewport);
+    SDL_RenderGetScale(renderer, &scale_x, &scale_y);
+    *x = (int) (*x / scale_x) - viewport.x;
+    *y = (int) (*y / scale_y) - viewport.y;
+}
+
+static struct point get_mouse_point(struct screen *screen) {
     int x;
     int y;
     SDL_GetMouseState(&x, &y);
+    convert_to_renderer_coordinates(screen->renderer, &x, &y);
     SDL_assert_release(x >= 0 && x < 0x10000 && y >= 0 && y < 0x10000);
     return (struct point) {
         .x = (Uint16) x,
@@ -204,7 +218,7 @@ void input_manager_process_mouse_wheel(struct input_manager *input_manager,
                                        const SDL_MouseWheelEvent *event) {
     struct position position = {
         .screen_size = input_manager->screen->frame_size,
-        .point = get_mouse_point(),
+        .point = get_mouse_point(input_manager->screen),
     };
     struct control_event control_event;
     if (mouse_wheel_from_sdl_to_android(event, position, &control_event)) {