mesa: EXT_dsa add selectorless matrix stack functions

Allows the legacy matrix stacks to be manipulated without disturbing the
matrix mode selector.

Adapted from a patch from Chris Forbes.
10 jobs for !829 with ext_dsa_matrix in 14 minutes and 5 seconds (queued for 24 seconds)
detached
Status Job ID Name Coverage
  Containers Build
passed #324819
debian

00:00:23

 
  Build+Test
passed #324821
meson-clang

00:07:44

passed #324825
meson-clover

00:08:18

passed #324824
meson-main

00:04:35

passed #324820
meson-swr-glvnd

00:04:13

passed #324826
meson-vulkan

00:03:07

passed #324828
scons-llvm

00:04:39

passed #324827
scons-nollvm

00:02:41

passed #324822
scons-swr

00:05:06

passed #324823
scons-win64

00:06:05