st/mesa: convert Mesa float formats to Gallium

st/mesa: convert Mesa float formats to Gallium

Squashed commit of the following:

Author: Marek Olšák <maraeo@gmail.com>

    st/mesa: require RGBA16F and RGBA32F to be renderable
    st/mesa: fix L32F and L16F format translation
    st/mesa: also convert the R/RG float formats

commit 49a9948b6a81b7d813304d081139d98e95ba5d1a
Author: Luca Barbieri <luca@luca-barbieri.com>
Date:   Fri Aug 20 10:36:17 2010 +0200

    mesa/st: enable ARB_texture_float if supported formats allow it

commit 7383632f7b6f9021b65f4973b7e7c99f0e8ce9b2
Author: Luca Barbieri <luca@luca-barbieri.com>
Date:   Tue Aug 24 21:00:46 2010 +0200

    mesa/st: support ARB_texture_float internal formats

commit 7c362cc06982586c2d29fac55f6bcc4bcd1550b5
Author: Luca Barbieri <luca@luca-barbieri.com>
Date:   Tue Aug 24 21:00:33 2010 +0200

    mesa/st: convert L/A/I floating point formats