If a texture is bound to a framebuffer before any data is set on it, the
texture does not have a type because it has never been bound.  This
generates an INVALID_OPERATION because the attachment type cannot be
validated.

Fixes:
 * Many generated GL_INVALID_OPERATIONs in chromium

BUG=angleproject:884

Change-Id: I4f36f80fecc5a772b6522f8f82e0be9d3758e524
Reviewed-on: https://chromium-review.googlesource.com/273324Tested-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Brandon Jones <bajones@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>