mesa: Add a rebase_swizzle parameter to _mesa_format_convert

mesa: Add a rebase_swizzle parameter to _mesa_format_convert

The new parameter allows callers to provide a rebase swizzle that
the function needs to use to match the requirements of the base
internal format involved. This is necessary when the source or
destination internal formats (depending on whether we are doing
the conversion for a pixel download or a pixel upload respectively)
do not match the base formats of the source or destination
formats of the conversion. This can happen when the driver does not
support the internal formats and uses a different format to store
pixel data internally.

For example, a texture upload from RGB to Luminance in a driver
that does not support textures with a Luminance format may decide
to store the Luminance data as RGBA. In this case we want to store
the RGBA values as (R,R,R,1). Following the same example, when we
download from that texture to RGBA we want to read (R,0,0,1). The
rebase_swizzle parameter allows these transforms to happen.

Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>