impr: Restore native macOS title bar double click gesture in borderless mode (#1689)

### Problem description

#### Problem 1
In borderless mode ImHex disables the standard macOS titlebar rendering
and input processing. As a result double clicking the titlebar does not
trigger the native macOS behavior set in `System Settings -> Desktop &
Dock -> Double-click a window's title bar to [Zoom/Minimize/Do
nothing]`.

#### Problem 2
The ImHex window shows up as blank/transparent when de-minimizing it
from the dock.

#### Problem 3
Widgets experience ghost hover inputs from the past position of the
cursor during live resizing.

### Implementation description
ImGui elements consume input events in the order they are drawn. As a
result by "drawing" an `InvisibleButton` over the content area of the
titlebar we can catch unprocessed clicks in the titlebar area.
Connecting this button's double clicks to the native window is then a
trivial endeavour.

The blank windows was caused by the rendering stack clearing the GL
buffer, but proceeding to draw nothing in it. I have short circuited
this path.

Ghost hover inputs were squelched by consistently moving the ImGui
cursor to `0, 0` during a live resize. The OS will dispatch a cursor
positioning event once the resizing ends, restoring normal behavior.

### Screenshots
N/A

### Additional things
N/A

---------

Co-authored-by: Nik <werwolv98@gmail.com>

plugins/builtin/source/content/window_decoration.cpp

@@ -392,6 +392,20 @@ namespace hex::plugin::builtin {
                 drawMenu();
                 drawTitleBar();
 
+                #if defined(OS_MACOS)
+                    if (ImHexApi::System::isBorderlessWindowModeEnabled()) {
+                        const auto windowSize = ImHexApi::System::getMainWindowSize();
+                        const auto menuUnderlaySize = ImVec2(windowSize.x, ImGui::GetCurrentWindowRead()->MenuBarHeight() * 1.5F);
+                        
+                        ImGui::SetCursorPos(ImVec2());
+                        
+                        // Drawing this button late allows widgets rendered before it to grab click events, forming an "input underlay"
+                        if (ImGui::InvisibleButton("##mainMenuUnderlay", menuUnderlaySize, ImGuiButtonFlags_PressedOnDoubleClick)) {
+                            macosHandleTitlebarDoubleClickGesture(window);
+                        }
+                    }
+                #endif
+                
                 ImGui::EndMainMenuBar();
             } else {
                 ImGui::PopStyleVar(2);