gallium: remove PIPE_CAP_S3TC, and add PIPE_CAP_DUAL_SOURCE_BLEND

gallium: remove PIPE_CAP_S3TC, and add PIPE_CAP_DUAL_SOURCE_BLEND

there's no point in keeping deprecated PIPE_CAPS around since there's no
stable interface (yet).
Reuse the enum for PIPE_CAP_DUAL_SOURCE_BLEND. Drivers advertizing this
will accept the pipe blendfactors with SRC1 in them and be able to do dual
source blending (src color from pixel shader output 0, and blendfactor with
SRC1 will come from output 1, only one render target may be bound when using
any of these blend factors).