mesa: Advertise EXT_float_blend in ES 3.0+ contexts.

mesa: Advertise EXT_float_blend in ES 3.0+ contexts.

This extension simply drops a draw time restriction:

    "Furthermore, an INVALID_OPERATION error is generated by
     DrawArrays and the other drawing commands defined in section
     2.8.3 (10.5 in ES 3.1) if blending is enabled (see below) and
     any draw buffer has 32-bit floating-point format components."

We never correctly enforced this restriction anyway, so we were
basically already implementing it.  We just need to advertise it
for our behavior to be correct.

The extension requires EXT_color_buffer_float, but we already enable
that via dummy_true.  So we can dummy_true this one as well.

Found while debugging WebGL conformance tests.  Does not fix any.

Reviewed-by: Tapani Pälli <tapani.palli@intel.com>