The current behavior is very ad-hoc and presents numerous issues to
downstream clients. Make this configuration process more straightforward
by adding an explicit clearing mode enum field to `pl_render_params`,
which the old override fields are specific instances of.

Notably, this could be extended in the future, e.g. by PL_CLEAR_GRADIENT
or whatever. (Although this can already be done as of right now by
setting both clear modes to PL_CLEAR_NONE and using blend_params on top
of a custom-drawn gradient)

The default of `PL_COLOR_CLEAR` for both is consistent with the current
status quo, and also presents the least surprising outcome when
combining transparent/opaque images with transparent/opaque
framebuffers. This way, the user only gets transparent images output if
they explicitly opt-in to it, either by setting
`background_transparency` to 1 (like `plplay` does), or by setting the
background clearing mode to `PL_CLEAR_SKIP`.

In particular, `border = PL_CLEAR_SKIP` will result in "garbage" pixels
out of the box, at least on implementations which don't pre-clear
framebuffers for you.

Supersedes: !637