Driver OPTIONS for graphics/mesa-*

Hey x11@ veterans.

While working through Mesa 25 [1] and 26 [2] updates I stumbled upon
the following problem
regarding OPTIONS handling. mesa-libs and mesa-dri are packages built
from the same codebase and we must take care to build both ports with
matching Meson options.

For example, the current version of graphics/mesa-libs passes
-Dgallium-drivers="r300" to Meson unconditionally, while
graphics/mesa-dri allows for changing gallium-drivers via the OPTIONS
mechanism. This worked so far, but starting with Mesa 25 upstream
followed the "megalib" approach, which made Meson options more
entangled. It requires building more gallium drivers in mesa-libs and
it is still possible to mess things up by selecting different PLATFORM
options.

We can try to stick with the current approach, like it is done in [1],
or make both graphics/mesa-libs and graphics/mesa-dri share the same
set of OPTIONS like it is done in [2]. Or we can just remove
all these OPTIONS altogether, which I like the most. In future, once
we get subpackages, these two ports can be merged into one and OPTIONS
reinstated, which would be an ideal solution.

What are your thoughts on this?

[1] https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=294948
[2] https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=294957