nv2a: Fix missing flat qualifier for inv_w param

`vtx_inv_w` should also be flat, since interpolating it causes incorrect values
for the flat shaded colors.

Fixes #1179

[Test](https://github.com/abaire/nxdk_pgraph_tests/blob/main/src/tests/shade_model_tests.cpp)
[HW results](https://github.com/abaire/nxdk_pgraph_tests_golden_results/wiki/Results-Shade_model)

hw/xbox/nv2a/shaders.c

@@ -262,7 +262,7 @@ static MString* generate_geometry_shader(
                        "void emit_vertex(int index, int provoking_index) {\n"
                        "  gl_Position = gl_in[index].gl_Position;\n"
                        "  gl_PointSize = gl_in[index].gl_PointSize;\n"
-                       "  vtx_inv_w = v_vtx_inv_w[index];\n"
+                       "  vtx_inv_w = v_vtx_inv_w[provoking_index];\n"
                        "  vtxD0 = v_vtxD0[provoking_index];\n"
                        "  vtxD1 = v_vtxD1[provoking_index];\n"
                        "  vtxB0 = v_vtxB0[provoking_index];\n"

hw/xbox/nv2a/shaders_common.h

@@ -24,7 +24,7 @@
 #include "debug.h"
 
 #define DEF_VERTEX_DATA(qualifier, in_out, prefix, suffix) \
-    "noperspective " in_out " float " prefix "vtx_inv_w" suffix ";\n" \
+    qualifier " " in_out " float " prefix "vtx_inv_w" suffix ";\n" \
     qualifier " " in_out " vec4 " prefix "vtxD0" suffix ";\n" \
     qualifier " " in_out " vec4 " prefix "vtxD1" suffix ";\n" \
     qualifier " " in_out " vec4 " prefix "vtxB0" suffix ";\n" \