gallium: add bitmap/drawpixels texcoord bias support

The state tracker will call pipe->get_paramf(PIPE_CAP_BITMAP_TEXCOORD_BIAS)
to get a bias factor for adjusting the texcoords used in bitmap/drawpixels.
This allows us to compensate for small differences in rasterization from
one device to another.

src/mesa/state_tracker/st_extensions.c

@@ -106,6 +106,9 @@ void st_init_limits(struct st_context *st)
 
    c->MaxTextureLodBias
       = pipe->get_paramf(pipe, PIPE_CAP_MAX_TEXTURE_LOD_BIAS);
+
+   st->bitmap_texcoord_bias
+      = pipe->get_paramf(pipe, PIPE_CAP_BITMAP_TEXCOORD_BIAS);
 }