X11: Clarify code flow for Clang static analysis

From upstream: https://github.com/glfw/glfw/commit/ef5220d6b10174d0f7cce965475e34527afcfbcf.

glfw/x11_window.c

@@ -1825,7 +1825,7 @@ int _glfwPlatformCreateWindow(_GLFWwindow* window,
                               const _GLFWctxconfig* ctxconfig,
                               const _GLFWfbconfig* fbconfig)
 {
-    Visual* visual;
+    Visual* visual = NULL;
     int depth;
 
     if (ctxconfig->client != GLFW_NO_API)
@@ -1851,8 +1851,7 @@ int _glfwPlatformCreateWindow(_GLFWwindow* window,
         }
     }
 
-    if (ctxconfig->client == GLFW_NO_API ||
-        ctxconfig->source == GLFW_OSMESA_CONTEXT_API)
+    if (!visual)
     {
         visual = DefaultVisual(_glfw.x11.display, _glfw.x11.screen);
         depth = DefaultDepth(_glfw.x11.display, _glfw.x11.screen);