[GPU] Async shader compilation for D3D12 and Vulkan

Adds async_shader_compilation cvar (default true) that forces new
pipelines to be created async from the main thread, skips draws entirely
on D3D12 and on vulkan replaces pixel shader with placeholder until the
real one is ready. Causes some visual artifacts on first load but
greatly reduces load times and stutter.

src/xenia/gpu/shaders/bytecode/vulkan_spirv/placeholder_ps.h

@@ -0,0 +1,47 @@
+// Generated with `xb buildshaders`.
+#if 0
+; SPIR-V
+; Version: 1.0
+; Generator: Khronos Glslang Reference Front End; 11
+; Bound: 9012
+; Schema: 0
+               OpCapability Shader
+          %1 = OpExtInstImport "GLSL.std.450"
+               OpMemoryModel Logical GLSL450
+               OpEntryPoint Fragment %main "main" %oC0
+               OpExecutionMode %main OriginUpperLeft
+               OpSource GLSL 460
+               OpName %main "main"
+               OpName %oC0 "oC0"
+               OpDecorate %oC0 Location 0
+       %void = OpTypeVoid
+       %1282 = OpTypeFunction %void
+      %float = OpTypeFloat 32
+    %v4float = OpTypeVector %float 4
+%_ptr_Output_v4float = OpTypePointer Output %v4float
+        %oC0 = OpVariable %_ptr_Output_v4float Output
+    %float_0 = OpConstant %float 0
+       %2938 = OpConstantComposite %v4float %float_0 %float_0 %float_0 %float_0
+       %main = OpFunction %void None %1282
+       %9011 = OpLabel
+               OpStore %oC0 %2938
+               OpReturn
+               OpFunctionEnd
+#endif
+
+const uint32_t placeholder_ps[] = {
+    0x07230203, 0x00010000, 0x0008000B, 0x00002334, 0x00000000, 0x00020011,
+    0x00000001, 0x0006000B, 0x00000001, 0x4C534C47, 0x6474732E, 0x3035342E,
+    0x00000000, 0x0003000E, 0x00000000, 0x00000001, 0x0006000F, 0x00000004,
+    0x0000161F, 0x6E69616D, 0x00000000, 0x00001238, 0x00030010, 0x0000161F,
+    0x00000007, 0x00030003, 0x00000002, 0x000001CC, 0x00040005, 0x0000161F,
+    0x6E69616D, 0x00000000, 0x00030005, 0x00001238, 0x0030436F, 0x00040047,
+    0x00001238, 0x0000001E, 0x00000000, 0x00020013, 0x00000008, 0x00030021,
+    0x00000502, 0x00000008, 0x00030016, 0x0000000D, 0x00000020, 0x00040017,
+    0x0000001D, 0x0000000D, 0x00000004, 0x00040020, 0x0000029A, 0x00000003,
+    0x0000001D, 0x0004003B, 0x0000029A, 0x00001238, 0x00000003, 0x0004002B,
+    0x0000000D, 0x00000A0C, 0x00000000, 0x0007002C, 0x0000001D, 0x00000B7A,
+    0x00000A0C, 0x00000A0C, 0x00000A0C, 0x00000A0C, 0x00050036, 0x00000008,
+    0x0000161F, 0x00000000, 0x00000502, 0x000200F8, 0x00002333, 0x0003003E,
+    0x00001238, 0x00000B7A, 0x000100FD, 0x00010038,
+};

src/xenia/gpu/shaders/placeholder.ps.glsl

@@ -0,0 +1,20 @@
+/**
+ ******************************************************************************
+ * Xenia : Xbox 360 Emulator Research Project                                 *
+ ******************************************************************************
+ * Copyright 2025 Ben Vanik. All rights reserved.                             *
+ * Released under the BSD license - see LICENSE in the root for more details. *
+ ******************************************************************************
+ */
+
+#version 460
+
+// Minimal placeholder pixel shader for pipeline hot-swap.
+// Used temporarily while the real shader compiles in the background.
+// Outputs transparent black to minimize visual disruption.
+
+layout(location = 0) out vec4 oC0;
+
+void main() {
+  oC0 = vec4(0.0, 0.0, 0.0, 0.0);
+}