This fixes an issue introduced in
340f531c71 where embedded OpenGL windows
would incorrectly have the WS_EX_LAYERED style bit set, which in turn
prevented them from displaying correctly.
Before the window-transparency refactoring, OpenGL windows would not
have the layered bit set because only peers with windowIsSemiTransparent
were created with the layered bit, and the layered bit was only updated
if the peer alpha was changed.
The new behaviour is to always set WS_EX_LAYERED if the peer's component
is non-opaque with no titlebar, or if the component has an alpha < 1.0f.
The OpenGLContext's placeholder component has no titlebar, so it must be
opaque in order to avoid setting the layered style bit.
735f99418c