Honor the surface's alphaMode on the Vulkan swapchain

[d3x0r]

SwapChainVk::ChooseConfig hardcoded VK_COMPOSITE_ALPHA_OPAQUE_BIT_KHR outside of Android, so a surface configured with a transparent alphaMode was validated and then composited as opaque even though the adapter advertised the mode as supported.

The Vulkan enum is now converted directly from the WebGPU one. Auto and unsupported modes are already resolved and validated in Surface.cpp against the capabilities PhysicalDeviceVk reports, so the Android-only fallback loop is gone and the remaining condition is a DAWN_CHECK. This is not an Android regression: PhysicalDeviceVk lists the modes in the same order the old loop preferred them (OPAQUE, PRE, POST, INHERIT) and Surface.cpp resolves Auto to alphaModes[0], so Auto reproduces the previous behaviour exactly.

ManualSurfaceTest gains a T key that creates a window with GLFW_TRANSPARENT_FRAMEBUFFER, and on such a window the clear-cycle render mode cycles a premultiplied alpha, so cycling alpha modes with A has a visible effect against the desktop.

The D3D side has been split into a separate PR as requested: https://github.com/d3x0r/dawn/tree/d3d-alphamode-composition

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[github-actions]

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[kainino0x]

Latest revision failed to import to Gerrit (our code review tool), due to the merge conflict. Could you update the PR?

[kainino0x]

Import worked! I think some of the comments on Gerrit still need to be addressed though.