sokol_app.h d3d11: add fallback if d3d11 debug device can't be created

If SOKOL_DEBUG is defined, the D3D11 device is attempted to be
created with D3D11_CREATE_DEVICE_DEBUG, which can fail if
the D3D11 debug layers are not installed or stopped working.
This fix adds a fallback path which logs a warning and retries
the device creation without the debug flag.

sokol_app.h

@@ -5911,6 +5911,34 @@ _SOKOL_PRIVATE void _sapp_d3d11_create_device_and_swapchain(void) {
         &feature_level,                 /* pFeatureLevel */
         &_sapp.d3d11.device_context);   /* ppImmediateContext */
     _SOKOL_UNUSED(hr);
+    #if defined(SOKOL_DEBUG)
+    if (!SUCCEEDED(hr)) {
+        // if initialization with D3D11_CREATE_DEVICE_DEBUG failes, this could be because the
+        // 'D3D11 debug layer' stopped working, indicated by the error message:
+        // ===
+        // D3D11CreateDevice: Flags (0x2) were specified which require the D3D11 SDK Layers for Windows 10, but they are not present on the system.
+        // These flags must be removed, or the Windows 10 SDK must be installed.
+        // Flags include: D3D11_CREATE_DEVICE_DEBUG
+        // ===
+        //
+        // ...just retry with the DEBUG flag switched off
+        SOKOL_LOG("sokol_app.h: D3D11CreateDeviceAndSwapChain() with D3D11_CREATE_DEVICE_DEBUG failed, retrying without debug flag.\n");
+        create_flags &= ~D3D11_CREATE_DEVICE_DEBUG;
+        hr = D3D11CreateDeviceAndSwapChain(
+            NULL,                           /* pAdapter (use default) */
+            D3D_DRIVER_TYPE_HARDWARE,       /* DriverType */
+            NULL,                           /* Software */
+            create_flags,                   /* Flags */
+            NULL,                           /* pFeatureLevels */
+            0,                              /* FeatureLevels */
+            D3D11_SDK_VERSION,              /* SDKVersion */
+            sc_desc,                        /* pSwapChainDesc */
+            &_sapp.d3d11.swap_chain,        /* ppSwapChain */
+            &_sapp.d3d11.device,            /* ppDevice */
+            &feature_level,                 /* pFeatureLevel */
+            &_sapp.d3d11.device_context);   /* ppImmediateContext */
+    }
+    #endif
     SOKOL_ASSERT(SUCCEEDED(hr) && _sapp.d3d11.swap_chain && _sapp.d3d11.device && _sapp.d3d11.device_context);
 
     // mimimize frame latency, disable Alt-Enter