Fix garbled text when display scalling > 300% (#3402)

* Fix garbled text when display scalling > 300%

On Windows, if your display scaling is greater than 300%,
the text rendered by imgui will be garbled. Allowing imgui
to do the scaling instead of making the font to large
corrects the issue.

* Only use modified scaling on Windows

And only if the system's scaling factor is greater than 300%.

---------

Co-authored-by: SoftFever <softfeverever@gmail.com>

src/slic3r/GUI/GLCanvas3D.cpp

@@ -6422,7 +6422,17 @@ void GLCanvas3D::_resize(unsigned int w, unsigned int h)
     m_last_w = w;
     m_last_h = h;
 
-    const float font_size = 1.5f * wxGetApp().em_unit();
+    float font_size = wxGetApp().em_unit();
+
+#ifdef _WIN32
+    // On Windows, if manually scaled here, rendering issues can occur when the system's Display
+    // scaling is greater than 300% as the font's size gets to be to large. So, use imgui font
+    // scaling instead (see: ImGuiWrapper::init_font() and issue #3401)
+    font_size *= (font_size > 30.0f) ? 1.0f : 1.5f;
+#else
+    font_size *= 1.5f;
+#endif
+
 #if ENABLE_RETINA_GL
     imgui->set_scaling(font_size, 1.0f, m_retina_helper->get_scale_factor());
 #else

src/slic3r/GUI/ImGuiWrapper.cpp

@@ -2663,6 +2663,15 @@ void ImGuiWrapper::init_font(bool compress)
         if (bold_font == nullptr) { throw Slic3r::RuntimeError("ImGui: Could not load deafult font"); }
     }
 
+#ifdef _WIN32
+    // Render the text a bit larger (see GLCanvas3D::_resize() and issue #3401), but only if the scale factor
+    // for the Display is greater than 300%.
+    if (wxGetApp().em_unit() > 30) {
+        default_font->Scale = 1.5f;
+        bold_font->Scale    = 1.5f;
+    }
+#endif
+
 #ifdef __APPLE__
     ImFontConfig config;
     config.MergeMode = true;