sdl_milton: Logic for stylus buttons with gui

Added clauses so that when a button on the stylus is pressed an appropriate
functionality is called based on the gui radio buttons in settings

src/sdl_milton.cc

@@ -214,6 +214,26 @@ panning_update(PlatformState* platform)
     }
 }
 
+static void stylus_buttons_exec_function(MiltonInput *milton_input, Milton *milton, int mode)
+{
+        switch (mode) {
+                case STYLUS_DEFAULT:
+                        break;
+                case STYLUS_ERASER:
+                        milton_input->mode_to_set = MiltonMode::ERASER;
+                        break;
+                case STYLUS_BRUSH:
+                        milton_input->mode_to_set = MiltonMode::PEN;
+                        break;
+                case STYLUS_DECB:
+                        for (int i=0;i<5;++i) milton_decrease_brush_size(milton);
+                        break;
+                case STYLUS_INCB:
+                        for (int i=0;i<5;++i) milton_increase_brush_size(milton);
+                        break;
+        }
+}
+
 MiltonInput
 sdl_event_loop(Milton* milton, PlatformState* platform)
 {
@@ -277,6 +297,9 @@ sdl_event_loop(Milton* milton, PlatformState* platform)
                     if(bit_lower && platform->lower_button == STYLUS_BUTTON_NOT_PRESSED) {
                         if(platform->lower_button == STYLUS_BUTTON_NOT_PRESSED) {
                                 platform->lower_button = STYLUS_BUTTON_PRESSED;
+                                stylus_buttons_exec_function(&milton_input,
+                                                             milton,
+                                                             milton->settings->lower_button);
                         }
                     }
 
@@ -287,6 +310,9 @@ sdl_event_loop(Milton* milton, PlatformState* platform)
                     if(bit_upper) {
                         if(platform->upper_button == STYLUS_BUTTON_NOT_PRESSED) {
                                 platform->upper_button = STYLUS_BUTTON_PRESSED;
+                                stylus_buttons_exec_function(&milton_input,
+                                                             milton,
+                                                             milton->settings->upper_button);
                         }
                     }