Re-anchor D3D9 swapchain dimensions across ctor and Reset paths

[BinqAdams]

Summary

Re-anchor m_originalWidth / m_originalHeight in three swapchain code paths so m_widthScale / m_heightScale settle correctly when CreateDevice and a subsequent Reset use different backbuffer dims.

Motivation

D3D9SwapChainEx::m_originalWidth / m_originalHeight is captured from the first CreateDevice's BackBuffer dims (in the ctor init list at d3d9_swapchain.cpp:327-328) and never updated, while NormalizePresentParameters recomputes m_widthScale = pp->BackBufferWidth / m_originalWidth on every Reset. When the host process creates the device at one resolution and later Resets to another (a common pattern for engines that initialize at a default size, then apply the user-selected resolution from a settings menu), m_widthScale becomes non-1.0 (or Inf when BackBufferWidth=0 triggers D3D9 auto-derive).

That scale multiplies every D3DVIEWPORT9 rect inside D3D9DeviceEx::SetViewport, so:

• Ratio mismatch (e.g. 640 → 1280 makes m_widthScale = 2.0): the Vulkan viewport overflows the backbuffer attachment — the visible result is the top-left quadrant of the rendered scene with the rest clipped.
• BackBufferWidth=0 ctor path: m_widthScale = NEW_BB / 0 = Inf, which propagates into SetViewport's per-rect multiply and produces a NaN/Inf VkViewport that fails CreateDevice with D3DERR_NOTAVAILABLE.

The D3D9SwapchainExternal subclass overrides Reset, so a fix limited to D3D9SwapChainEx::Reset doesn't reach the bridge / RemixAPI path.

What Changed

src/d3d9/d3d9_swapchain.cpp: re-anchor m_originalWidth / m_originalHeight in three sites:

1. D3D9SwapChainEx ctor BB=0 path. After NormalizePresentParameters fills in the auto-derived backbuffer dims, if the init list captured 0 from the caller, re-anchor to the post-Normalize values and reset m_widthScale / m_heightScale to 1.0.
2. D3D9SwapChainEx::Reset. Re-anchor to the new backbuffer dims before NormalizePresentParameters runs, so m_widthScale settles at 1.0 for the new size.
3. D3D9SwapchainExternal::Reset (bridge / RemixAPI path). Mirror the same fix; the override would otherwise leave m_originalWidth pinned to the first CreateDevice's value.

The DXVK_RESOLUTION_WIDTH / DXVK_RESOLUTION_HEIGHT upscale feature is preserved by gating each re-anchor on those env vars being empty — when they're set, the engine-supplied dims are intentionally overwritten by the env values to produce an upscale ratio, and re-anchoring would defeat that.

Net diff: src/d3d9/d3d9_swapchain.cpp only, +77.

Testing

Verified behavior on three reproducers:

• Engine that creates at a hardcoded default resolution then Resets to the user-selected resolution: pre-fix produced the top-left-quadrant clipping symptom in 2D content (menus, video, HUD); post-fix renders correctly across the new resolution.
• CreateDevice with BackBufferWidth=BackBufferHeight=0 (D3D9 auto-derive from HWND): pre-fix returned D3DERR_NOTAVAILABLE from the first SetViewport due to Inf/NaN; post-fix succeeds with m_widthScale = m_heightScale = 1.0.
• DXVK_RESOLUTION_WIDTH / DXVK_RESOLUTION_HEIGHT env vars set: re-anchor is skipped, intentional upscale ratio is preserved.