Fix borderless-windowed mode being always on top

[auwsmit]

Don't pass a monitor to glfwSetWindowMonitor(). This avoids setting the flag internal to glfw which keeps the window always on top.

BorderlessWindowed still works fine, and is properly detected on my two monitors at different resolutions.

Addresses this issue: #5854, but only for borderless FS.

A workaround for regular aka "exclusive" Fullscreen is mentioned in that thread, just add this to raylib's loop:

if (!IsWindowFocused() && IsWindowFullscreen())
{
    MinimizeWindow();
}