If extensions such as OES_texture_float_linear are not exposed but the
native drive still supports them, we don't want to bind the native texture
and instead bind a texture that is known to sample like the incomplete
texture.

BUG=angleproject:1959

Change-Id: I610dbd93bb566c8eb2166488a7494b74da4aa327
Reviewed-on: https://chromium-review.googlesource.com/465090
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>