Fix bug with mouse wheel on export combo

- Fix `gui_wants_capture_mouse` to check the value before we call
  ImGui::NewFrame.  It seems to work better like that.

- Ignore wheel event if the UI wants to capture the mouse.

src/goxel.c

@@ -826,6 +826,7 @@ static int on_zoom(const gesture_t *gest, void *user)
     double zoom;
     camera_t *camera = get_camera();
 
+    if (gui_want_capture_mouse()) return 0;
     zoom = (gest->pos[1] - gest->last_pos[1]) / 10.0;
     mat4_itranslate(camera->mat, 0, 0,
             -camera->dist * (1 - pow(1.1, -zoom)));
@@ -901,7 +902,7 @@ void goxel_mouse_in_view(const float viewport[4], const inputs_t *inputs,
         tool_iter(goxel.tool, &painter, viewport);
     }
 
-    if (inputs->mouse_wheel) {
+    if (inputs->mouse_wheel && !gui_want_capture_mouse()) {
         mat4_itranslate(camera->mat, 0, 0,
                 -camera->dist * (1 - pow(1.1, -inputs->mouse_wheel)));
         camera->dist *= pow(1.1, -inputs->mouse_wheel);

src/gui.cpp

@@ -165,6 +165,7 @@ typedef struct gui_t {
     int     scrolling;
 
     int     can_move_window;
+    bool    want_capture_mouse;
 
     int     is_row;
     float   item_size;
@@ -604,6 +605,7 @@ static void gui_iter(inputs_t *inputs)
     ImGuiIO& io = ImGui::GetIO();
     ImGuiStyle& style = ImGui::GetStyle();
 
+    gui->want_capture_mouse = io.WantCaptureMouse;
     io.DisplaySize = ImVec2((float)goxel.screen_size[0],
                             (float)goxel.screen_size[1]);
 
@@ -1928,9 +1930,7 @@ bool gui_icons_grid(int nb, const gui_icon_info_t *icons, int *current)
 
 bool gui_want_capture_mouse(void)
 {
-    gui_init();
-    ImGuiIO& io = ImGui::GetIO();
-    return io.WantCaptureMouse;
+    return gui && gui->want_capture_mouse;
 }
 
 bool gui_context_menu_begin(const char *label)