Vulkan: match OpenGL's depth bias in the shadow map pass (#7944)

OpenGL's shadow pass calls gr_zbias(-1024), which sets a polygon offset of
factor 1, units 1024 (gropengltnl.cpp, gropengl.cpp). The Vulkan shadow pass
set a constant bias of -1024 with no slope term: the opposite sign, on a
depth buffer that uses the same less-than test. Use OpenGL's values.

Co-authored-by: Dustinspace217 <[email protected]>

code/graphics/vulkan/VulkanDraw.cpp

@@ -748,7 +748,7 @@ void VulkanDrawManager::renderShadowDraw(gr_buffer_handle ubo_handle, size_t ubo
 	}
 
 	stateTracker->bindPipeline(pipeline, pipelineManager->getPipelineLayout());
-	stateTracker->setDepthBias(-1024.0f, 0.0f);
+	stateTracker->setDepthBias(1024.0f, 1.0f); // GL shadow pass: gr_zbias(-1024) = polygon offset factor 1, units 1024
 
 	// Bind descriptor sets: shadowMapData (per-draw) and shadowCascadeParams
 	// (bound separately, per-frame, via shadow_cascade_params_bind) both live in