Add GLFW_X11_XCB_VULKAN_SURFACE init hint

This hint controls whether GLFW will try to use VK_KHR_xcb_surface
before falling back to VK_KHR_xlib_surface.

Closes #1793.

include/GLFW/glfw3.h

@@ -1250,6 +1250,11 @@ extern "C" {
  *  macOS specific [init hint](@ref GLFW_COCOA_MENUBAR_hint).
  */
 #define GLFW_COCOA_MENUBAR          0x00051002
+/*! @brief X11 specific init hint.
+ *
+ *  X11 specific [init hint](@ref GLFW_X11_XCB_VULKAN_SURFACE_hint).
+ */
+#define GLFW_X11_XCB_VULKAN_SURFACE 0x00052001
 /*! @} */
 
 #define GLFW_DONT_CARE              -1

src/init.c

@@ -57,7 +57,10 @@ static _GLFWinitconfig _glfwInitHints =
     {
         GLFW_TRUE,  // macOS menu bar
         GLFW_TRUE   // macOS bundle chdir
-    }
+    },
+    {
+        GLFW_TRUE,  // X11 XCB Vulkan surface
+    },
 };
 
 // Terminate the library
@@ -298,6 +301,9 @@ GLFWAPI void glfwInitHint(int hint, int value)
         case GLFW_COCOA_MENUBAR:
             _glfwInitHints.ns.menubar = value;
             return;
+        case GLFW_X11_XCB_VULKAN_SURFACE:
+            _glfwInitHints.x11.xcbVulkanSurface = value;
+            return;
     }
 
     _glfwInputError(GLFW_INVALID_ENUM,

src/internal.h

@@ -248,6 +248,9 @@ struct _GLFWinitconfig
         GLFWbool  menubar;
         GLFWbool  chdir;
     } ns;
+    struct {
+        GLFWbool  xcbVulkanSurface;
+    } x11;
 };
 
 // Window configuration

src/x11_init.c

@@ -813,11 +813,15 @@ static GLFWbool initExtensions(void)
                               XkbGroupStateMask, XkbGroupStateMask);
     }
 
+    if (_glfw.hints.init.x11.xcbVulkanSurface)
+    {
 #if defined(__CYGWIN__)
-    _glfw.x11.x11xcb.handle = _glfw_dlopen("libX11-xcb-1.so");
+        _glfw.x11.x11xcb.handle = _glfw_dlopen("libX11-xcb-1.so");
 #else
-    _glfw.x11.x11xcb.handle = _glfw_dlopen("libX11-xcb.so.1");
+        _glfw.x11.x11xcb.handle = _glfw_dlopen("libX11-xcb.so.1");
 #endif
+    }
+
     if (_glfw.x11.x11xcb.handle)
     {
         _glfw.x11.x11xcb.GetXCBConnection = (PFN_XGetXCBConnection)