meta: Don't ask for floating point textures if not ARB_texture_float.

meta: Don't ask for floating point textures if not ARB_texture_float.

I was promoting to float for ARB_color_buffer_float unclamped, which
failed when ARB_texture_float wasn't present.  Since the metaops don't
need results outside of [0,1] when not drawing to a floating point
destination, they can just use a fixed point texture when floating
point destinations are impossible.

Fixes regression in fdo23670-depth_test when --enable-texture-float is
not present.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=36473