mmdevapi: Reorder a couple of functions to avoid forward declarations and make MMDevice_SetPropValue() static.

Unless the driver implements ARB_ES2_compatibility / GL4.1, FBO completeness
depends on what read / draw buffers are set.

For the benefit of depth-only draws and clears, that would otherwise fail FBO
completeness, unless the driver supports ARB_ES2_compatibility.

Anything that isn't explicitly specified is set to GL_NONE by
glDrawBuffersARB().

It's trivial, and allows the rest of the code to assume at least 1 render
target is present.

winex11: Move the SendMessage(WM_SYSCOMMAND, SC_MAXIMIZE/SC_RESTORE) call after the window extents check.

This will allow us the get rid of the swapchain refcounting hacks in d3d9 in
particular. This is similar to the way we handle resources that are still in
use by a stateblock, but aren't referenced anywhere by the application.

They're mostly the same, except for the GL version also destroying its
contexts, and being a bit more careful about the order in which backbuffers
are destroyed.