Only build glslang if Vulkan or Direct3D 12 rendering is enabled

glslang isn't needed for OpenGL rendering, which includes the web export.
This reduces the web release export template's `.wasm` size by about 20 KB,
since web builds use `vulkan=no`.

modules/glslang/config.py

@@ -1,5 +1,7 @@
 def can_build(env, platform):
-    return True
+    # glslang is only needed when Vulkan or Direct3D 12-based renderers are available,
+    # as OpenGL doesn't use glslang.
+    return env["vulkan"] or env["d3d12"]
 
 
 def configure(env):