nv2a: Fix pixel center offset problem

This addresses the difference in pixel center between OpenGL and the DirectX 8
approach used by the hardware.

Note that the resultant behavior still does not fully match the hardware, where
screen-space pixel coordinates below (x + 0.5625) => x, but it is close enough
to fix issues with surface-target rendering.

hw/xbox/nv2a/pgraph.c

@@ -4074,6 +4074,13 @@ static void pgraph_shader_update_constants(PGRAPHState *pg,
         glUniform2f(binding->clip_range_loc, zclip_min, zclip_max);
     }
 
+    if (binding->gl_viewport_size_loc != -1) {
+        unsigned int vp_width = pg->surface_binding_dim.width,
+                vp_height = pg->surface_binding_dim.height;
+        pgraph_apply_scaling_factor(pg, &vp_width, &vp_height);
+        glUniform2f(binding->gl_viewport_size_loc, vp_width, vp_height);
+    }
+
     /* Clipping regions */
     for (i = 0; i < 8; i++) {
         uint32_t x = pg->regs[NV_PGRAPH_WINDOWCLIPX0 + i * 4];

hw/xbox/nv2a/shaders.c

@@ -706,6 +706,7 @@ static MString *generate_vertex_shader(const ShaderState state,
 "\n"
 "uniform vec2 clipRange;\n"
 "uniform vec2 surfaceSize;\n"
+"uniform vec2 glViewportSize;\n"
 "\n"
 /* All constants in 1 array declaration */
 "uniform vec4 c[" stringify(NV2A_VERTEXSHADER_CONSTANTS) "];\n"
@@ -1046,6 +1047,7 @@ ShaderBinding* generate_shaders(const ShaderState state)
         snprintf(tmp, sizeof(tmp), "c[%d]", i);
         ret->vsh_constant_loc[i] = glGetUniformLocation(program, tmp);
     }
+    ret->gl_viewport_size_loc = glGetUniformLocation(program, "glViewportSize");
     ret->surface_size_loc = glGetUniformLocation(program, "surfaceSize");
     ret->clip_range_loc = glGetUniformLocation(program, "clipRange");
     ret->fog_color_loc = glGetUniformLocation(program, "fogColor");

hw/xbox/nv2a/shaders.h

@@ -111,6 +111,7 @@ typedef struct ShaderBinding {
     GLint bump_offset_loc[NV2A_MAX_TEXTURES];
     GLint tex_scale_loc[NV2A_MAX_TEXTURES];
 
+    GLint gl_viewport_size_loc;
     GLint surface_size_loc;
     GLint clip_range_loc;
 

hw/xbox/nv2a/vsh.c

@@ -839,14 +839,14 @@ void vsh_translate(uint16_t version,
     mstring_append(body,
         /* the shaders leave the result in screen space, while
          * opengl expects it in clip space.
-         * TODO: the pixel-center co-ordinate differences should handled
          */
         "  oPos.x = 2.0 * (oPos.x - surfaceSize.x * 0.5) / surfaceSize.x;\n"
         "  oPos.y = -2.0 * (oPos.y - surfaceSize.y * 0.5) / surfaceSize.y;\n"
     );
     if (z_perspective) {
         mstring_append(body, "  oPos.z = oPos.w;\n");
     }
+
     mstring_append(body,
         /* Map the clip range into clip space so z is clipped correctly.
          * Note this makes the values in the depth buffer wrong. This should be
@@ -869,5 +869,10 @@ void vsh_translate(uint16_t version,
         "  }\n"
     );
 
+    /* Fix up the pixel-center difference between OpenGL and DirectX 8
+     * TODO: This still does not entirely match HW behavior.
+     * Pixels should be floored below 0.5 + 1/16 = 0.5625
+     */
+    mstring_append(body,
+        "  oPos.xy += (vec2(-1.0f, 1.0f) / glViewportSize.xy) * oPos.w;\n");
 }
-