spirv: Add an execution environment to the options

Also updates gl_spirv to pick the right one.  At the moment nothing
uses it, but upcoming functionality part of ARB_gl_spirv will use it,
and we also later can be more assertful when handling certain features
for each of the execution environments.

Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Acked-by: Karol Herbst <kherbst@redhat.com>

src/compiler/spirv/nir_spirv.h

@@ -50,7 +50,15 @@ enum nir_spirv_debug_level {
    NIR_SPIRV_DEBUG_LEVEL_ERROR,
 };
 
+enum nir_spirv_execution_environment {
+   NIR_SPIRV_VULKAN = 0,
+   NIR_SPIRV_OPENCL,
+   NIR_SPIRV_OPENGL,
+};
+
 struct spirv_to_nir_options {
+   enum nir_spirv_execution_environment environment;
+
    /* Whether or not to lower all workgroup variable access to offsets
     * up-front.  This means you will _shared intrinsics instead of _var
     * for workgroup data access.

src/mesa/main/glspirv.c

@@ -211,6 +211,7 @@ _mesa_spirv_to_nir(struct gl_context *ctx,
    }
 
    const struct spirv_to_nir_options spirv_options = {
+      .environment = NIR_SPIRV_OPENGL,
       .lower_workgroup_access_to_offsets = true,
       .lower_ubo_ssbo_access_to_offsets = true,
       .caps = ctx->Const.SpirVCapabilities