x11: Initialize m_target of EGL platform surface texture

We need to initialize m_target; otherwise the surface texture won't be
bound to GL_TARGET_2D.

src/plugins/platforms/x11/standalone/eglbackend.cpp

@@ -155,6 +155,7 @@ EglPixmapTexturePrivate::EglPixmapTexturePrivate(EglPixmapTexture *texture, EglB
     : q(texture)
     , m_backend(backend)
 {
+    m_target = GL_TEXTURE_2D;
 }
 
 EglPixmapTexturePrivate::~EglPixmapTexturePrivate()