Mesa: Add support for GL_OES_texture_*float* extensions.

Mesa: Add support for GL_OES_texture_*float* extensions.

This patch series adds support for following GLES2 Texture Float extensions:
1)GL_OES_texture_float,
2)GL_OES_texture_half_float,
3)GL_OES_texture_float_linear,
4)GL_OES_texture_half_float_linear.

This patch adds basic infrastructure and needed boolean flags to advertise
support for these extensions, by default the support is disabled. Next patch
in the series introduces support for HALF_FLOAT_OES token.

v4: take assert away and make valid_filter_for_float conditional (Tapani),
    fix the alphabetical order (Emil)

Signed-off-by: Kevin Rogovin <kevin.rogovin@intel.com>
Signed-off-by: Kalyan Kondapally <kalyan.kondapally@intel.com>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>