Fix Multithreaded Compilation of Shader Cache on OpenGL (#3540)

This was broken by the Vulkan changes - OpenGL was building host caches at boot on one thread, which is very notably slower than when it is multithreaded.

This was caused by trying to get the program binary immediately after compilation started, which blocks. Now it does it after compilation has completed.

Ryujinx.Graphics.Gpu/Shader/DiskCache/ParallelDiskCacheLoader.cs

@@ -434,7 +434,10 @@ namespace Ryujinx.Graphics.Gpu.Shader.DiskCache
                     _needsHostRegen = true;
                 }
 
-                _programList.Add(entry.ProgramIndex, (entry.CachedProgram, entry.BinaryCode));
+                // Fetch the binary code from the backend if it isn't already present.
+                byte[] binaryCode = entry.BinaryCode ?? entry.CachedProgram.HostProgram.GetBinary();
+
+                _programList.Add(entry.ProgramIndex, (entry.CachedProgram, binaryCode));
                 SignalCompiled();
             }
             else if (entry.IsBinary)
@@ -502,7 +505,8 @@ namespace Ryujinx.Graphics.Gpu.Shader.DiskCache
                 IProgram hostProgram = _context.Renderer.CreateProgram(shaderSources, shaderInfo);
                 CachedShaderProgram program = new CachedShaderProgram(hostProgram, compilation.SpecializationState, compilation.Shaders);
 
-                byte[] binaryCode = _context.Capabilities.Api == TargetApi.Vulkan ? ShaderBinarySerializer.Pack(shaderSources) : hostProgram.GetBinary();
+                // Vulkan's binary code is the SPIR-V used for compilation, so it is ready immediately. Other APIs get this after compilation.
+                byte[] binaryCode = _context.Capabilities.Api == TargetApi.Vulkan ? ShaderBinarySerializer.Pack(shaderSources) : null;
 
                 EnqueueForValidation(new ProgramEntry(program, binaryCode, compilation.ProgramIndex, compilation.IsCompute, isBinary: false));
             }