mesa: delete framebuffer texture attachment sampler views

mesa: delete framebuffer texture attachment sampler views

When a context is destroyed the destroy_tex_sampler_cb makes sure that all the
sampler views created by that context are destroyed.
This is done by walking the ctx->Shared->TexObjects hash table.

In a multiple context environment the texture can be deleted by a different context,
so it will be removed from the TexObjects table and will prevent the above mechanism
to work.
This can result in an assertion in st_save_zombie_sampler_view because the
sampler_view owns a reference to a destroyed context.

This issue occurs in blender 2.80.

This commit fixes this by explicitly releasing sampler_view created by the destroyed
context for all texture attachments.

Fixes: 593e36f956 (st/mesa: implement "zombie" sampler views (v2))
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=110944
Signed-off-by: Marek Olšák <marek.olsak@amd.com>