pico: add helper to make sure overlay is rendered

Sometimes we're going to lock out rendering for a while...

32blit-pico/overlay.cpp

diff --git a/32blit-pico/overlay.cpp b/32blit-pico/overlay.cpp
index 31a2cf3..aec4133 100644
--- a/32blit-pico/overlay.cpp
+++ b/32blit-pico/overlay.cpp
@@ -8,6 +8,7 @@
 
 static std::string message;
 static uint32_t progress_value = 0, progress_total = 0;
+static bool render_flag = false;
 
 static void overlay_render(uint32_t time) {
   // TODO: don't want to do this if we add a menu to avoid the problems the stm32 firmware has...
@@ -22,6 +23,8 @@
     uint32_t progress_width = float(progress_value) / progress_total * (screen.bounds.w - 10);
     screen.rectangle(Rect(5, screen.bounds.h - 10, progress_width, 5));
   }
+
+  render_flag = false;
 }
 
 void set_render_overlay_enabled(bool enabled) {
@@ -42,3 +45,10 @@
   progress_value = value;
   progress_total = total;
 }
+
+void overlay_try_render(bool force) {
+  render_flag = force;
+  do {
+    update_display(blit::now());
+  } while(render_flag);
+}

32blit-pico/overlay.hpp

diff --git a/32blit-pico/overlay.hpp b/32blit-pico/overlay.hpp
index f6aab2b..3f22de5 100644
--- a/32blit-pico/overlay.hpp
+++ b/32blit-pico/overlay.hpp
@@ -4,3 +4,5 @@
 
 void set_overlay_message(std::string_view text);
 void set_overlay_progress(uint32_t value, uint32_t total);
+
+void overlay_try_render(bool force);