Shadows: Added experimental texture-based shadows (stripped of dynamic tex config and back-end code)

(merged 10 commits, removing dynamic tex config, moved tex config to internal structs, removed back-end changes)
(squashed 2025-06-11, fixed conflict, doesn't build as-is)
Shadows: Added IMGUI_HAS_SHADOWS
Shadows: Demo code in Custom Rendering section. Added AddShadowRectFilled() variant. BeginMainMenuBar() disable shadows.
Shadows: Added initial version of convex shape shadow code.
(+stripped out of original polygon generation demo, moved to imgui_dev)
Shadows: Tweak demo to use AddShadowCircle() functions + fix warnings.
 (+ stripped old polygon generation code from commits)
Shadows: Convex shape shadow improvement/fixes
- Fixed pixel cracking on convex shadow edges
- Added convex shadow offset support
- Fixed some convex shadow fringing issues
- Added convex shadow demo code
Shadows: Added ImDrawShadowFlags, simplified API surface, reordered parameters
+ fix minor warnings
+ removed NGon variant for now.
Shadows: Re-added AddShadowNGon().
Shadows: Shallow styling tweaks and renaming for consistency.
Shadows: Fixes + two fixes for PVS Studio static analyzer.
Shadows: Fix for latest, reuse ImDrawList, remove ImDrawShadowFlags.
Shadows: Fix to support for colored-only font atlas. (4202)
Shadows: Fix broken shadows due to zero-clear added in 9417acc2.
Shadows: remove use of GetWindowContentRegionMax().
Shadows: WIP fixes for latest. (broken UV caching)
Shadows: WIP for latest (refreshing UV coordinates).

imgui_widgets.cpp

@@ -9070,10 +9070,14 @@ bool ImGui::BeginViewportSideBar(const char* name, ImGuiViewport* viewport_p, Im
     }
 
     window_flags |= ImGuiWindowFlags_NoTitleBar | ImGuiWindowFlags_NoResize | ImGuiWindowFlags_NoMove;
+
+    // Create window
+    PushStyleColor(ImGuiCol_WindowShadow, ImVec4(0, 0, 0, 0));
     PushStyleVar(ImGuiStyleVar_WindowRounding, 0.0f);
     PushStyleVar(ImGuiStyleVar_WindowMinSize, ImVec2(0, 0)); // Lift normal size constraint
     bool is_open = Begin(name, NULL, window_flags);
     PopStyleVar(2);
+    PopStyleColor();
 
     return is_open;
 }