[rcore][win32] stop lying to the OS about when our window is updated

Instead of calling BeginPaint/EndPaint in WM_PAINT which signals to the
OS that our window content is updated, now when we encounter the WM_PAINT
message instead we return back to the raylib application which will
trigger it to render a new frame. We've replaced the call to BeginPaint
and EndPaint with a call to ValidateRect in SwapBuffers, which, means
we're now correctly telling the OS when our window content is actually
up-to-date.

Note that this doesn't fix the window content not being updated during
a window resize/move beacuse thos have their own message loop which
doesn't return early when it's time to paint.

src/platforms/rcore_desktop_win32.c

@@ -1439,6 +1439,11 @@ void SwapScreenBuffer(void)
         LogFail("SwapBuffers", GetLastError());
         abort();
     }
+
+    if (!ValidateRect(global_hwnd, NULL)) {
+        LogFail("ValidateRect", GetLastError());
+        abort();
+    }
 }
 
 double GetTime(void)
@@ -1551,6 +1556,8 @@ void PollInputEvents(void)
 
     MSG msg;
     while (PeekMessageW(&msg, NULL, 0, 0, PM_REMOVE)) {
+        if (msg.message == WM_PAINT)
+            return;
         TranslateMessage(&msg);
         DispatchMessageW(&msg);
     }
@@ -1778,12 +1785,6 @@ static LRESULT WndProc2(HWND hwnd, UINT msg, WPARAM wparam, LPARAM lparam, Flags
         }
         return 0;
     }
-    case WM_PAINT: {
-        PAINTSTRUCT ps;
-        BeginPaint(hwnd, &ps);
-        EndPaint(hwnd, &ps);
-        return 0;
-    }
     case WM_SETCURSOR:
         if (LOWORD(lparam) == HTCLIENT) {
             SetCursor(CORE.Input.Mouse.cursorHidden ? NULL : LoadCursorW(NULL, IDC_ARROW));