Calculate DEPTH_SCALE correctly for polygon offset.

src/mesa/drivers/dri/unichrome/via_context.c

@@ -356,6 +356,7 @@ viaCreateContext(const __GLcontextModes *mesaVis,
        vmesa->depth_max = (GLfloat)0xffff;
        vmesa->depth_clear_mask = 0xf << 28;
        vmesa->ClearDepth = 0xffff;
+       vmesa->polygon_offset_scale = 1.0 / vmesa->depth_max;
        break;
     case 24:
        vmesa->hasDepth = GL_TRUE;
@@ -370,6 +371,7 @@ viaCreateContext(const __GLcontextModes *mesaVis,
        vmesa->have_hw_stencil = GL_TRUE;
        vmesa->stencilBits = mesaVis->stencilBits;
        vmesa->stencil_clear_mask = 0x1 << 28;
+       vmesa->polygon_offset_scale = 2.0 / vmesa->depth_max;
        break;
     case 32:
        vmesa->hasDepth = GL_TRUE;
@@ -380,6 +382,7 @@ viaCreateContext(const __GLcontextModes *mesaVis,
        vmesa->depth_clear_mask = 0;
        vmesa->ClearDepth = 0xffffffff;
        vmesa->depth_clear_mask = 0xf << 28;
+       vmesa->polygon_offset_scale = 2.0 / vmesa->depth_max;
        break;
     default:
        assert(0); 

src/mesa/drivers/dri/unichrome/via_context.h

@@ -103,6 +103,7 @@ struct via_context_t {
    GLuint depth_clear_mask;
    GLuint stencil_clear_mask;
    GLfloat depth_max;
+   GLfloat polygon_offset_scale;
 
     GLubyte    *dma;
     viaRegion tex;

src/mesa/drivers/dri/unichrome/via_tris.c

@@ -222,7 +222,7 @@ static struct {
     } while (0)
 
 
-#define DEPTH_SCALE (1.0 / 0xffff)
+#define DEPTH_SCALE vmesa->polygon_offset_scale
 #define UNFILLED_TRI unfilled_tri
 #define UNFILLED_QUAD unfilled_quad
 #define VERT_X(_v) _v->v.x