WebglGraphicsDevice.initializeRenderState reset its cached active texture
unit to 0 but never issued gl.activeTexture(TEXTURE0). On a fresh context
this is harmless (WebGL defaults to unit 0), but when a device is created on
a context previously used by a now-destroyed device - e.g. a second
GraphicsDevice on the same canvas, as the Examples code-editor hot-reload
does - the context can be left on a non-zero active unit. The cache then
disagreed with GL state, so activeTexture(0)'s redundancy check skipped the
call and the first texture bound to unit 0 (such as a directional shadow
map) silently landed on the wrong unit. The sampler2DShadow then read a
non-comparison texture and returned 0, so the whole scene read as shadowed
and rendered dark after hot-reloading an example with a shadow-casting light.

Force the active unit to match the cache during render-state init. This is
a long-standing issue (reproducible back to at least Jan 2025), not a recent
regression.

Fixes #8893

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>