zink: allow software rendering only if selected

In environments where 3D acceleration is not available, like in a VM,
the behavior before commit 8cd44b88 ("egl: add automatic zink
fallback loading between hw and sw drivers") was to fallback to swrast.

This was the output of `eglinfo` in that situation:

    $ eglinfo
    [...]
    Wayland platform:
    EGL driver name: swrast
    OpenGL core profile renderer: llvmpipe (LLVM 17.0.4, 256 bits)

However, after commit 8cd44b88 ("egl: add automatic zink fallback
loading between hw and sw drivers") Zink support is tested before
falling back to swrast.
Since the system doesn't support 3D acceleration, Zink + software
rendering is used instead of swrast causing issues like the ones
described in #10146.

In this case, `eglinfo` prints:

    $ eglinfo
    [...]
    Wayland platform:
    EGL driver name: zink
    OpenGL core profile renderer: zink Vulkan 1.3(llvmpipe (LLVM 17.0.4,
    256 bits) (MESA_LLVMPIPE))

This patch ensures that Zink + software rendering is used only when the
user opts-in by setting `LIBGL_ALWAYS_SOFTWARE` or `D3D_ALWAYS_SOFTWARE`
and swrast is used otherwise.

After the patch, the output of `eglinfo` is identical to the one before
the regression:

    $ eglinfo
    [...]
    Wayland platform:
    EGL driver name: swrast
    OpenGL core profile renderer: llvmpipe (LLVM 17.0.4, 256 bits)

Resolves: mesa/mesa#10146


Fixes: 8cd44b88 ("egl: add automatic zink fallback loading between
hw and sw drivers")

Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>
Signed-off-by: José Expósito <jexposit@redhat.com>
Part-of: <mesa/mesa!26220>