gallium: tweak coords in u_gen_mipmap code

src/gallium/auxiliary/util/u_gen_mipmap.c

@@ -773,23 +773,23 @@ set_vertex_data(struct gen_mipmap_state *ctx, float width, float height)
 {
    void *buf;
 
-   ctx->vertices[0][0][0] = 0.0f; /*x*/
-   ctx->vertices[0][0][1] = 0.0f; /*y*/
+   ctx->vertices[0][0][0] = -0.5f; /*x*/
+   ctx->vertices[0][0][1] = -0.5f; /*y*/
    ctx->vertices[0][1][0] = 0.0f; /*s*/
    ctx->vertices[0][1][1] = 0.0f; /*t*/
 
-   ctx->vertices[1][0][0] = width; /*x*/
-   ctx->vertices[1][0][1] = 0.0f;  /*y*/
+   ctx->vertices[1][0][0] = width - 0.5f; /*x*/
+   ctx->vertices[1][0][1] = -0.5f;  /*y*/
    ctx->vertices[1][1][0] = 1.0f; /*s*/
    ctx->vertices[1][1][1] = 0.0f; /*t*/
 
-   ctx->vertices[2][0][0] = width;
-   ctx->vertices[2][0][1] = height;
+   ctx->vertices[2][0][0] = width - 0.5f;
+   ctx->vertices[2][0][1] = height - 0.5f;
    ctx->vertices[2][1][0] = 1.0f;
    ctx->vertices[2][1][1] = 1.0f;
 
-   ctx->vertices[3][0][0] = 0.0f;
-   ctx->vertices[3][0][1] = height;
+   ctx->vertices[3][0][0] = -0.5f;
+   ctx->vertices[3][0][1] = height - 0.5f;
    ctx->vertices[3][1][0] = 0.0f;
    ctx->vertices[3][1][1] = 1.0f;