gallium: Add a cap for offset_units_unscaled

D3D9 has a different behaviour for depth bias.

For OGL/D3D1X, the depth bias unit is the
minimal resolvable value for the depth buffer,
which depends on the format (and has different
behaviour for float depth buffers).

For D3D9, the depth bias unit is 1.0f.

Signed-off-by: Axel Davy <axel.davy@ens.fr>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>

src/gallium/docs/source/cso/rasterizer.rst

@@ -127,6 +127,12 @@ offset_tri
 
 offset_units
     Specifies the polygon offset bias
+offset_units_unscaled
+    Specifies the unit of the polygon offset bias. If false, use the
+    GL/D3D1X behaviour. If true, offset_units is a floating point offset
+    which isn't scaled (D3D9). Note that GL/D3D1X behaviour has different
+    formula whether the depth buffer is unorm or float, which is not
+    the case for D3D9.
 offset_scale
     Specifies the polygon offset scale
 offset_clamp

src/gallium/docs/source/screen.rst

@@ -343,6 +343,8 @@ The integer capabilities:
 * ``PIPE_CAP_TGSI_VOTE``: Whether the ``VOTE_*`` ops can be used in shaders.
 * ``PIPE_CAP_MAX_WINDOW_RECTANGLES``: The maxium number of window rectangles
   supported in ``set_window_rectangles``.
+* ``PIPE_CAP_POLYGON_OFFSET_UNITS_UNSCALED``: If true, the driver implements support
+  for ``pipe_rasterizer_state::offset_units_unscaled``.
 
 
 .. _pipe_capf: