configure: add upper version constraint on libplacebo

Libplacebo v6 substantially reworks the public API related to color
spaces, tone mapping, and more - and also requires GLSL 130 as a minimum
version, as well as access to OpenGL-backed textures to do any
nontrivial form of color mapping.

Not only does this result in compilation failures, but it also totally
prevents basic operation. These problems can be addressed on VLC v4, but
backporting all of that to VLC v3 would be prohitively costly. As such,
simply prevent VLC v3 from being combined with libplacebo v6,
intentionallly or otherwise.