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.