d2d1: Implement ID2D1DeviceContext6::DrawSvgDocument via librsvg This patch implements SVG document rendering in Wine's d2d1 by loading librsvg and Cairo on the unix side via the unixlib thunk mechanism. How it works: * d2d_svg_document_create reads the SVG data from the provided IStream, passes it to the unix side where librsvg parses it into an RsvgHandle * DrawSvgDocument renders the SVG into a pixel buffer using Cairo, creates a staging D2D1 bitmap from the result, and draws it into the device context * The unix lib is only initialized and used on 64-bit builds; 32-bit builds return E_NOTIMPL FPU note: Cairo's fsin leaves the x87 PE (Precision Exception) flag set after rendering. On systems where threads inherit FPU state from their parent with unmasked exceptions (CW=0x0040), this causes SIGFPE in newly created threads. fninit is called after each render to reset x87 state. This was observed in Solid Edge 2026 using Gecko-based UI components. Testing: A conformance test is included that verifies basic SVG document creation and rendering into a bitmap render target. Tested on Windows 10 1809+ and Wine. Windows 10 1507 and earlier do not support CreateSvgDocument and are skipped gracefully. Windows 8.1 does not support ID2D1DeviceContext6 and is also skipped. [Test Bot Job for this patch](https://testbot.winehq.org/JobDetails.pl?Key=162021) -- https://gitlab.winehq.org/wine/wine/-/merge_requests/10156