glsl/standalone: init shader stage in init_gl_program()

Otherwise lima standalone compiler fails when trying to compile fragment
shader with:

lima_compiler: ../src/compiler/nir/nir.c:55: nir_shader_create: Assertion `si->stage == stage' failed

Reviewed-by: Qiang Yu <yuq825@gmail.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Signed-off-by: Vasily Khoruzhick <anarsoul@gmail.com>
17 jobs for !1703 with glsl-standalone-init-shader-stage in 15 minutes (queued for 2 seconds)
detached
Status Job ID Name Coverage
  Containers Build
passed #512094
debian

00:00:27

 
  Build+Test
passed #512103
meson-arm64

00:02:34

passed #512102
meson-armhf

00:04:40

passed #512096
meson-clang

00:07:14

passed #512100
meson-clover

00:10:57

passed #512104
meson-i386

00:03:22

passed #512099
meson-main

00:08:26

passed #512095
meson-swr-glvnd

00:05:50

passed #512101
meson-vulkan

00:05:32

passed #512106
scons-llvm

00:02:53

passed #512105
scons-nollvm

00:03:54

passed #512097
scons-swr

00:06:33

passed #512098
scons-win64

00:07:25

 
  Test
passed #512107
test-llvmpipe-gles2 1/4

00:02:33

passed #512108
test-llvmpipe-gles2 2/4

00:03:35

passed #512109
test-llvmpipe-gles2 3/4

00:03:14

passed #512110
test-llvmpipe-gles2 4/4

00:02:49