mesa: Allow setting GL_TEXTURE_MAX_LEVEL to 0 with GL_TEXTURE_RECTANGLE.

mesa: Allow setting GL_TEXTURE_MAX_LEVEL to 0 with GL_TEXTURE_RECTANGLE.

Currently, we raise an error when doing this which breaks a conformance
test from the OpenGL samples pack. Even if this is a bit silly it is not
an error.

From http://www.opengl.org/wiki/Rectangle_Texture:

"Rectangle textures contain exactly one image; they cannot have mipmaps.
Therefore, any texture parameters that depend on LODs are irrelevant
when used with rectangle textures; attempting to set these parameters to
any value other than 0 will result in an error."

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=76496

Reviewed-by: Brian Paul <brianp@vmware.com>