Fix hooked video darkening for *_SRGB-backbuffer games

The host sampled the shared backbuffer copy through a view typed exactly like the
game's backbuffer. For games whose backbuffer is an *_SRGB format (e.g. Life is
Strange: Before the Storm -- confirmed R8G8B8A8_UNORM_SRGB / fmt 29 via the hook
log), the GPU decoded sRGB->linear on the sample, and the host then wrote those
linear values straight to its plain-UNORM swapchain with no re-encode, so the
mirror came out noticeably darker than the game.

Sample the copy as the plain-UNORM sibling of the format (srgb_to_unorm) so the
bytes pass through unchanged -- matching what WGC already does. The UNORM and
*_SRGB formats share a typeless group, so CopyResource from the producer's sRGB
texture into the host's UNORM copy is allowed. Non-sRGB formats are unaffected.

Adds srgb_format_test locking the mapping. All 6 tests pass.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

host/src/capture/shared_texture.cpp

@@ -1,5 +1,6 @@
 #include "capture/shared_texture.hpp"
 
+#include "capture/dxgi_format.hpp"
 #include "coop/protocol.hpp"
 #include "coop/shared_memory.hpp"
 
@@ -57,12 +58,15 @@ bool SharedTextureSource::reopen(unsigned long pid, const VideoShareView& share)
 	}
 
 	// Private copy we sample from, so we only hold the keyed mutex during the copy.
+	// Use the plain-UNORM sibling of the (possibly sRGB) backbuffer format so the
+	// SRV passes the bytes through without an sRGB->linear decode that would darken
+	// the mirror (CopyResource is allowed within the shared typeless group).
 	D3D11_TEXTURE2D_DESC desc{};
 	desc.Width = share.width;
 	desc.Height = share.height;
 	desc.MipLevels = 1;
 	desc.ArraySize = 1;
-	desc.Format = static_cast<DXGI_FORMAT>(share.format);
+	desc.Format = srgb_to_unorm(static_cast<DXGI_FORMAT>(share.format));
 	desc.SampleDesc.Count = 1;
 	desc.Usage = D3D11_USAGE_DEFAULT;
 	desc.BindFlags = D3D11_BIND_SHADER_RESOURCE;