perf(webgl2): allocate immutable texture storage once, upload with texSubImage2D

[Endymi0n74]

What

The WebGL2 renderer uploaded the video frame with texImage2D() on every frame, which reallocates the texture's GPU storage each time. This PR:

1. allocates the storage once with texStorage2D(gl.TEXTURE_2D, 1, gl.RGB8, w, h) and uploads per frame with texSubImage2D() on that immutable storage,
2. keeps the texture bound across frames (no per-frame bindTexture — the binding now happens inside allocateStorage, once per allocation),
3. recreates the texture when the video resolution changes (immutable storage can't be resized),
4. fixes a black-screen bug: the current TS source has no texStorage2D call, but the earlier fork iteration used gl.RGB — an unsized format that is invalid for texStorage2D (INVALID_ENUM → storage never allocated → per-frame texSubImage2D uploads fail → black renderer). Sized gl.RGB8 is required; texSubImage2D keeps gl.RGB.

Why

texImage2D on a per-frame basis reallocates GPU storage every frame (~×2.1 the cost of a texSubImage2D into an immutable storage on the measured driver). The upload path dominates the frame cost.

Safety

• Initial allocation is deferred until the canvas (or the first valid video frame) has a size > 0 — updateFrame allocates on the first frame where videoWidth/Height > 0 if no storage exists yet.
• On resolution change the old texture is removed from resources and deleted before re-allocating (no leak, no stale binding).
• destroy() nulls texture after the resource loop.
• Visual output is unchanged (same shader, same colors — RGB8 is the sized equivalent of RGB).

Measurement

Real GPU harness (640×360 VP9 test video, WebGL2 via ANGLE/D3D11, instrumented GL counters, seed protocol):

Metric	before	after	Δ
Video upload (µs/upload, tight loop)	~42–78	~8–12	×5.5
updateFrame total wall (ms/frame)	~0.043–0.074	~0.011–0.020	×2.8
GL calls per frame	texImage2D + drawArrays	texSubImage2D + drawArrays (0 texImage2D, 0 bindTexture)	—
Draw (rasterization)	10.2 µs	10.2 µs	unchanged (same shader)