On Windows 7, when GLFW framebuffer transparency and the DWM are enabled
but DWM transparency is disabled (i.e. when the Transparency setting is
disabled under Personalization > Color), the contents of the framebuffer
is combined with the last frame using additive blending instead of
replacing the previous contents.
This commit limits GLFW framebuffer transparency on Windows 7 to when
DWM transparency is enabled, removing the previous workaround of setting
a layered window color key that led to rendering artifacts.
Fixes#1512.
05dd2fa298
