[client] egl: dynamically import glBufferStorageEXT

On some implementations (e.g. llvmpipe), the function can only be queried via
eglGetProcAddress.

client/src/egl_dynprocs.c

@@ -40,6 +40,8 @@ void egl_dynProcsInit(void)
     eglGetProcAddress("glDebugMessageCallback");
   g_egl_dynProcs.glDebugMessageCallbackKHR = (glDebugMessageCallback_t)
     eglGetProcAddress("glDebugMessageCallbackKHR");
+  g_egl_dynProcs.glBufferStorageEXT = (glBufferStorageEXT_t)
+    eglGetProcAddress("glBufferStorageEXT");
 };
 
 #endif