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 49a9948b
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 7383632f
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 7c362cc0
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