Highlight selected objects (#12115)

# Description

The current bounding box selection display loses all usefulness once the furthest apart objects are selected.

This is especially noticeable on grid patterns, where nested unselected elements become indistinguishable after selecting all.

This PR implements a highlighting method that visually brightens individually selected objects.

# Screenshots/Recordings/Graphs

<img width="1845" height="829" alt="image" src="https://github.com/user-attachments/assets/a92a7176-182c-41e2-869e-badb220dfee2" />
Current 2.3.1: All but some objects are selected. Can you tell which ones?

<img width="2169" height="1042" alt="image" src="https://github.com/user-attachments/assets/c6c721d5-4f03-4c23-beff-b0acc0601568" />
Proposed PR: Slight lightening of selected objects

## Tests

Built on windows, tested with various colors and non-printable

src/slic3r/GUI/3DScene.cpp

@@ -188,6 +188,58 @@ void GLVolume::load_render_colors()
     RenderColor::colors[RenderCol_Model_Unprintable] = GUI::ImGuiWrapper::to_ImVec4(GLVolume::UNPRINTABLE_COLOR);
 }
 
+ColorRGBA GLVolume::brighten_color(const ColorRGBA& color, float multiplier)
+{
+    // Convert RGB to HSL, increase lightness, convert back
+
+    float r = color.r(), g = color.g(), b = color.b();
+
+    // RGB to HSL conversion
+    float max_val = std::max({r, g, b});
+    float min_val = std::min({r, g, b});
+    float l = (max_val + min_val) / 2.0f;
+    float h = 0.0f, s = 0.0f;
+
+    if (max_val != min_val) {
+        float delta = max_val - min_val;
+        s = l > 0.5f ? delta / (2.0f - max_val - min_val) : delta / (max_val + min_val);
+
+        if (max_val == r)
+            h = (g - b) / delta + (g < b ? 6.0f : 0.0f);
+        else if (max_val == g)
+            h = (b - r) / delta + 2.0f;
+        else
+            h = (r - g) / delta + 4.0f;
+        h /= 6.0f;
+    }
+
+    // Increase lightness by a fixed amount (0.25)
+    // Ensures even saturated colors become visibly brighter
+    l = std::min(l + 0.25f, 1.0f);
+
+    // HSL to RGB conversion
+    auto hue_to_rgb = [](float p, float q, float t) {
+        if (t < 0.0f) t += 1.0f;
+        if (t > 1.0f) t -= 1.0f;
+        if (t < 1.0f / 6.0f) return p + (q - p) * 6.0f * t;
+        if (t < 1.0f / 2.0f) return q;
+        if (t < 2.0f / 3.0f) return p + (q - p) * (2.0f / 3.0f - t) * 6.0f;
+        return p;
+    };
+
+    if (s == 0.0f) {
+        r = g = b = l; // achromatic (gray)
+    } else {
+        float q = l < 0.5f ? l * (1.0f + s) : l + s - l * s;
+        float p = 2.0f * l - q;
+        r = hue_to_rgb(p, q, h + 1.0f / 3.0f);
+        g = hue_to_rgb(p, q, h);
+        b = hue_to_rgb(p, q, h - 1.0f / 3.0f);
+    }
+
+    return ColorRGBA(r, g, b, color.a());
+}
+
 GLVolume::GLVolume(float r, float g, float b, float a)
     : m_sla_shift_z(0.0)
     , m_sinking_contours(*this)
@@ -253,16 +305,28 @@ void GLVolume::set_render_color()
             set_render_color(outside ? SELECTED_OUTSIDE_COLOR : SELECTED_COLOR);
         else if (disabled)
         */
-        if (disabled)
-            set_render_color(DISABLED_COLOR);
+        // Determine base color first
+        ColorRGBA base_color;
+
+        if (disabled) {
+            base_color = DISABLED_COLOR;
+        }
 #ifdef ENABLE_OUTSIDE_COLOR
-        else if (is_outside && shader_outside_printer_detection_enabled)
-            set_render_color(OUTSIDE_COLOR);
+        else if (is_outside && shader_outside_printer_detection_enabled) {
+            base_color = OUTSIDE_COLOR;
+        }
 #endif
         else {
-            //to make black not too hard too see
-            ColorRGBA new_color = adjust_color_for_rendering(color);
-            set_render_color(new_color);
+            // to make black not too hard too see
+            base_color = adjust_color_for_rendering(color);
+        }
+
+        // Apply selection brightening AFTER determining base color
+        if (selected && !disabled) {
+            set_render_color(brighten_color(base_color, 1.25f));
+        }
+        else {
+            set_render_color(base_color);
         }
     }
 
@@ -276,7 +340,11 @@ void GLVolume::set_render_color()
 
     //BBS set unprintable color
     if (!printable) {
-        render_color = UNPRINTABLE_COLOR;
+        if (selected) {
+            render_color = brighten_color(UNPRINTABLE_COLOR, 1.25f);
+        } else {
+            render_color = UNPRINTABLE_COLOR;
+        }
     }
 
     //BBS set invisible color

src/slic3r/GUI/3DScene.hpp

@@ -96,6 +96,7 @@ public:
 
     static void update_render_colors();
     static void load_render_colors();
+    static ColorRGBA brighten_color(const ColorRGBA& color, float multiplier = 1.25f);
 
     static float explosion_ratio;
     static float last_explosion_ratio;