fizzy-3.4.16-1

GPU: Let each driver decide whether it can claim a transparent window

SDL_ClaimWindowForGPUDevice refused every window created with
SDL_WINDOW_TRANSPARENT, but two of the three drivers already handle one:
the Cocoa Metal view is created non-opaque for a transparent window
(SDL_cocoametalview.m), and the Vulkan driver picks a premultiplied or
inherited composite alpha for one (VULKAN_INTERNAL_CreateSwapchain). The
generic check made both unreachable.

The refusal moves into D3D12_ClaimWindow, the one driver whose swapchain
(a flip-model swapchain created for the HWND) ignores alpha.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VoomBKpLopEWb9fUR4Qi46