GeomeryShaderGen: Set gl_ClipDistance on Vulkan as well as OpenGL

[Pokechu22]

Fixes https://bugs.dolphin-emu.org/issues/12548.

Both Vulkan and OpenGL use gl_ClipDistance when backend depth clamping is enabled. However, the geometry shader was only copying it for OpenGL, not for Vulkan. Since stereoscopic 3D uses a geometry shader, this was most visible there, but I suspect it could also affect line/point effects (I was unable to confirm this though).

Here's the relevant vertex shader code (skipping the non-geomerty-shader path):

dolphin/Source/Core/VideoCommon/VertexShaderGen.cpp

Lines 561 to 568 in 6dae404

	if (api_type == APIType::OpenGL || api_type == APIType::Vulkan)
	{
	if (host_config.backend_geometry_shaders)
	{
	AssignVSOutputMembers(out, "vs", "o", uid_data->numTexGens, host_config);
	}
	else
	{

dolphin/Source/Core/VideoCommon/VertexShaderGen.cpp

Lines 582 to 599 in 6dae404

	}
	if (host_config.backend_depth_clamp)
	{
	out.Write("gl_ClipDistance[0] = clipDist0;\n"
	"gl_ClipDistance[1] = clipDist1;\n");
	}
	// Vulkan NDC space has Y pointing down (right-handed NDC space).
	if (api_type == APIType::Vulkan)
	out.Write("gl_Position = float4(o.pos.x, -o.pos.y, o.pos.z, o.pos.w);\n");
	else
	out.Write("gl_Position = o.pos;\n");
	}
	else // D3D
	{
	out.Write("return o;\n");
	}

[iwubcode]

LGTM. Tested and confirmed that the bug was fixed, didn't notice any regressions