mesa/st: Disable OES_texture_3D for PIPE_CAP_MAX_TEXTURE_3D_LEVELS==0.

This will be used for vc4, where incorrectly exposing 3D textures accounts
for most of the GLES2 conformance failures it has.  This leaves
EXT_texture3d exposed in the (already non-conformant) GL2.1 support it
exposes, which has always been a best-effort thing.

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/17350>

src/compiler/glsl/standalone_scaffolding.cpp

@@ -218,6 +218,7 @@ void initialize_context_to_defaults(struct gl_context *ctx, gl_api api)
 
    ctx->Extensions.OES_EGL_image_external = true;
    ctx->Extensions.OES_standard_derivatives = true;
+   ctx->Extensions.OES_texture_3D = true;
 
    ctx->Extensions.EXT_gpu_shader4 = true;
    ctx->Extensions.EXT_shader_integer_mix = true;