wglShareLists will only succeed when the shared context is not currently
active on another thread, but it is difficult to ensure that the GL
thread is paused/inactive at any given time - in particular, it's very
easy to accidentally introduce deadlocks where the main thread waits for
the render thread's lock while the render thread waits for the
messagemanager lock.
The simplest way to ensure that no other thread has activated the shared
context is to share the contexts directly on the render thread itself.
This avoids an issue where, when attaching an OpenGLContext as a child
of a window rendered with Direct2D, the area covered by the OpenGL
renderer would display as a white rectangle until the context had
drawn a frame. This only affected the Direct2D renderer; when the parent
window used the software renderer, no white rectangle was shown.
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.
