fix: flipY in webgpu renderpass

[xiaoiver]

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• New feature
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[gemini-code-assist]

Code Review

This pull request implements _viewportFlipHeight to use the active render target height for Y-flipping, ensuring correct rendering for offscreen passes. Feedback suggests simplifying the attachment level logic, avoiding string-based access to private members, and removing redundant nullish coalescing.

[gemini-code-assist]

The logic for determining level0 can be simplified. In TypeScript, accessing an index beyond the array length returns undefined, and undefined || 0 correctly evaluates to 0. This avoids the need for an explicit length check.

Suggested change

	const level0 =
	this.gfxColorAttachmentLevel.length > 0
	? this.gfxColorAttachmentLevel[0] || 0
	: 0;
	const level0 = this.gfxColorAttachmentLevel[0] || 0;

[gemini-code-assist]

Accessing the private member swapChainHeight via string indexing (this.device['swapChainHeight']) is a hack that bypasses TypeScript's visibility checks. This makes the code fragile and dependent on internal implementation details of the Device_WebGPU class. It is recommended to expose this property through a public getter or a dedicated method in the Device_WebGPU class.

[gemini-code-assist]

Avoid using string indexing to access private members. Additionally, since _viewportFlipHeight is initialized in all branches of the setRenderPassDescriptor method (which is called at the start of every render pass), this nullish coalescing fallback is likely redundant.