This field was doing double duty between marking the alpha mode as
unknown and marking the alpha channel as absent, with inconsistent
results. In particular, pl_renderer always inferred the alpha channel if
present in the texture, leaving users with no options for easily
manually disabling it.

This patch adds PL_ALPHA_NONE to rectify the situation, allowing API
users to set `target.repr.alpha = PL_ALPHA_NONE` to explicitly disable
alpha blending even when the target supports it.

See-Also: !637