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 Name Job ID Coverage
  Containers Build
passed debian #512094

00:00:27

 
  Build+Test
passed meson-arm64 #512103

00:02:34

passed meson-armhf #512102

00:04:40

passed meson-clang #512096

00:07:14

passed meson-clover #512100

00:10:57

passed meson-i386 #512104

00:03:22

passed meson-main #512099

00:08:26

passed meson-swr-glvnd #512095

00:05:50

passed meson-vulkan #512101

00:05:32

passed scons-llvm #512106

00:02:53

passed scons-nollvm #512105

00:03:54

passed scons-swr #512097

00:06:33

passed scons-win64 #512098

00:07:25

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

00:02:33

passed test-llvmpipe-gles2 2/4 #512108

00:03:35

passed test-llvmpipe-gles2 3/4 #512109

00:03:14

passed test-llvmpipe-gles2 4/4 #512110

00:02:49