Fast path when rebinding the same texture in single context environment

If there is no shared context, there is no purpose in rebinding the same
texture.  In some artificial tests this improves performance 10% - 30%.
(cherry picked from commit 7f8000db8b)

src/mesa/main/texobj.c

@@ -891,6 +891,7 @@ _mesa_BindTexture( GLenum target, GLuint texName )
    struct gl_texture_unit *texUnit = &ctx->Texture.Unit[unit];
    struct gl_texture_object *newTexObj = NULL, *defaultTexObj = NULL;
    GLint targetIndex;
+   GLboolean early_out = GL_FALSE;
    ASSERT_OUTSIDE_BEGIN_END(ctx);
 
    if (MESA_VERBOSE & (VERBOSE_API|VERBOSE_TEXTURE))
@@ -944,6 +945,17 @@ _mesa_BindTexture( GLenum target, GLuint texName )
 
    assert(valid_texture_object(newTexObj));
 
+   _glthread_LOCK_MUTEX(ctx->Shared->Mutex);
+   if ((ctx->Shared->RefCount == 1)
+       && (newTexObj == texUnit->CurrentTex[targetIndex])) {
+      early_out = GL_TRUE;
+   }
+   _glthread_UNLOCK_MUTEX(ctx->Shared->Mutex);
+
+   if (early_out) {
+      return;
+   }
+
    /* flush before changing binding */
    FLUSH_VERTICES(ctx, _NEW_TEXTURE);