TransformFeedback: buffer allocation and usage requirements are implicit

Two TransformFeedback setup requirements are currently implicit — neither is discoverable from the API, and both fail in ways that do not point at the cause. Found while building the graphics/transform-feedback-separate example (#9131).

1. A buffer written only by the GPU still needs initial data

A vertex buffer that transform feedback fills, and that nothing ever uploads to, must still be constructed with data — otherwise it never gets GPU storage:

// broken - GL buffer is never allocated
const instances = new VertexBuffer(device, format, count, { usage: BUFFER_GPUDYNAMIC });

// works
const instances = new VertexBuffer(device, format, count, {
    usage: BUFFER_GPUDYNAMIC,
    data: new Float32Array(count * 4)   // never read, only forces allocation
});

Cause is in the VertexBuffer constructor — without data it allocates CPU-side storage and stops there, so setData/unlock never runs and no bufferData call is made:

const initialData = options?.data;
if (initialData) {
    this.setData(initialData);
} else {
    this.storage = new ArrayBuffer(this.numBytes);
}

The failure is remote from the cause. gl.getBufferParameter(ARRAY_BUFFER, BUFFER_SIZE) returns 0, bindBufferBase binds nothing, and beginTransformFeedback returns GL_INVALID_OPERATION because a binding point the program writes has no buffer. Nothing mentions the buffer that was never allocated, and there is no assert. It took a while to track down from that starting point.

Passing zeroed data purely to trigger allocation is also a real cost — count * 4 floats uploaded and thrown away for a buffer whose contents are immediately overwritten by the GPU.

Possible fixes:

• Allocate GPU storage in the constructor regardless of whether data was supplied
• Or add an explicit way to say "allocate, do not upload"
• Or, at minimum, assert in setTransformFeedbackBuffers that each buffer has GPU storage, so the error names the actual problem

2. BUFFER_GPUDYNAMIC is effectively required, but silently worked around

TransformFeedback re-uploads the buffer to change its usage when it was not created with BUFFER_GPUDYNAMIC:

if (usage === BUFFER_GPUDYNAMIC && outVB.usage !== usage) {
    // have to recreate input buffer with other usage
    gl.bindBuffer(gl.ARRAY_BUFFER, outVB.impl.bufferId);
    gl.bufferData(gl.ARRAY_BUFFER, outVB.storage, gl.DYNAMIC_COPY);
}

It works, but silently — the caller gets an extra full re-upload per buffer with no indication, and nothing tells them the usage they chose was inappropriate. A Debug.warnOnce naming the buffer would make the cost visible and steer people to BUFFER_GPUDYNAMIC up front.

Notes

Neither blocks anything — both are documented as workarounds in playcanvas/developer-site#1115. Raising them so the docs can eventually drop those caveats rather than enshrine them.