lighting: alpha should be multiplied, not summed

In the second color channel, the alpha component of the diffuse material
should not be added, but multiplied with the input from the previous
stage, because the OpenGL spec says that the value of alpha should be
taken from the alpha component of the color material's diffuse value.

This fixes the rendering of glass parts in Neverball.

src/gc_gl.c

@@ -2042,7 +2042,7 @@ bool _ogx_setup_render_stages()
     if (glparamstate.lighting.enabled) {
         LightMasks light_mask = prepare_lighting();
 
-        GXColor color_zero = { 0, 0, 0, 0 };
+        GXColor color_black = { 0, 0, 0, 255 };
         GXColor color_gamb = gxcol_new_fv(glparamstate.lighting.globalambient);
 
         _ogx_gpu_resources->tevstage_first += 2;
@@ -2110,9 +2110,9 @@ bool _ogx_setup_render_stages()
                        light_mask.ambient_mask | light_mask.specular_mask , GX_DF_NONE, GX_AF_SPEC);
         GX_SetChanAmbColor(GX_COLOR0A0, color_gamb);
 
-        // Color1 channel: Multiplies the light raster result with the vertex color. Ambient is set to register (which is zero)
+        // Color1 channel: Multiplies the light raster result with the vertex color. Ambient is set to register (which is black)
         GX_SetChanCtrl(GX_COLOR1A1, GX_TRUE, GX_SRC_REG, vert_color_src, light_mask.diffuse_mask, GX_DF_CLAMP, GX_AF_SPOT);
-        GX_SetChanAmbColor(GX_COLOR1A1, color_zero);
+        GX_SetChanAmbColor(GX_COLOR1A1, color_black);
 
         // STAGE 0: ambient*vert_color -> cprev
         // In data: d: Raster Color, a: emission color
@@ -2128,7 +2128,7 @@ bool _ogx_setup_render_stages()
         // STAGE 1: diffuse*vert_color + cprev -> cprev
         // In data: d: Raster Color a: CPREV
         GX_SetTevColorIn(GX_TEVSTAGE1, GX_CC_CPREV, GX_CC_ZERO, GX_CC_ZERO, GX_CC_RASC);
-        GX_SetTevAlphaIn(GX_TEVSTAGE1, GX_CA_APREV, GX_CA_ZERO, GX_CA_ZERO, GX_CA_RASA);
+        GX_SetTevAlphaIn(GX_TEVSTAGE1, GX_CA_ZERO, GX_CA_APREV, GX_CA_RASA, GX_CA_ZERO);
         // Operation: Sum a + d
         GX_SetTevColorOp(GX_TEVSTAGE1, GX_TEV_ADD, GX_TB_ZERO, GX_CS_SCALE_1, GX_TRUE, GX_TEVPREV);
         GX_SetTevAlphaOp(GX_TEVSTAGE1, GX_TEV_ADD, GX_TB_ZERO, GX_CS_SCALE_1, GX_TRUE, GX_TEVPREV);