gl-renderer: Ignore INVALID modifier

gl-renderer: Ignore INVALID modifier

If the user has passed an INVALID modifier, it's because there is no
applicable modifier, and the buffer layout should be determined by a
magic side-channel call (e.g. bo_get_tiling). If the modifier is
INVALID, don't try to pass it through to EGL, but just drop it.

On the other hand, if a modifier _is_ explicitly specified and we don't
have the modifiers extension, then refuse to import the buffer.

Signed-off-by: Daniel Stone <daniels@collabora.com>