TEV: do not call GX_SetTevColor with GX_REGPREV

It turns out that while this works on Dolphin, the GX_TEVPREV register
cannot be written to, on a real Wii. Therefore, write on a GX_TEVREGx
register, and allocate them dynamically.

src/gc_gl.c

@@ -2118,8 +2118,11 @@ bool _ogx_setup_render_stages()
 
         // STAGE 0: ambient*vert_color -> cprev
         // In data: d: Raster Color, a: emission color
-        GX_SetTevColor(GX_TEVPREV, ecol);
-        GX_SetTevColorIn(GX_TEVSTAGE0, GX_CC_CPREV, GX_CC_ZERO, GX_CC_ZERO, GX_CC_RASC);
+        u8 emission_reg = _ogx_gpu_resources->tevreg_first++;
+        GX_SetTevColor(GX_TEVREG0 + emission_reg, ecol);
+        /* Multiply by two because there are alpha registers in between */
+        GX_SetTevColorIn(GX_TEVSTAGE0, GX_CC_C0 + emission_reg * 2,
+                         GX_CC_ZERO, GX_CC_ZERO, GX_CC_RASC);
         GX_SetTevAlphaIn(GX_TEVSTAGE0, GX_CA_ZERO, GX_CA_ZERO, GX_CA_ZERO, GX_CA_RASA);
         // Operation: Pass d
         GX_SetTevColorOp(GX_TEVSTAGE0, GX_TEV_ADD, GX_TB_ZERO, GX_CS_SCALE_1, GX_TRUE, GX_TEVPREV);

src/gpu_resources.c

@@ -52,6 +52,9 @@ static void resources_init(OgxGpuResources *resources)
     resources->kcolor_first = 0;
     resources->kcolor_end = GX_KCOLOR_MAX;
 
+    resources->tevreg_first = 0;
+    resources->tevreg_end = GX_MAX_TEVREG - 1; /* we exclude GX_TEVPREV */
+
     resources->texcoord_first = 0;
     resources->texcoord_end = GX_MAXCOORD;
 

src/gpu_resources.h

@@ -57,6 +57,8 @@ typedef struct {
     uint8_t tevstage_end;
     uint8_t kcolor_first;
     uint8_t kcolor_end;
+    uint8_t tevreg_first;
+    uint8_t tevreg_end;
     uint8_t texcoord_first;
     uint8_t texcoord_end;
     uint8_t pnmtx_first;

src/stencil.c

@@ -451,11 +451,14 @@ static bool draw_op(uint16_t op,
     glparamstate.dirty.bits.dirty_color_update = 1;
 
     u8 stage = GX_TEVSTAGE0 + _ogx_gpu_resources->tevstage_first++;
-    GX_SetTevColor(GX_TEVPREV, drawColor);
+    u8 tevreg_index = _ogx_gpu_resources->tevreg_first++;
+    GX_SetTevColor(GX_TEVREG0 + tevreg_index, drawColor);
     GX_SetTevOrder(stage, GX_TEXCOORDNULL, GX_TEXMAP_DISABLE, GX_COLOR0A0);
     /* Pass the constant color */
-    GX_SetTevColorIn(stage, GX_CC_ZERO, GX_CC_ZERO, GX_CC_ZERO, GX_CC_CPREV);
-    GX_SetTevAlphaIn(stage, GX_CA_ZERO, GX_CA_ZERO, GX_CA_ZERO, GX_CA_APREV);
+    GX_SetTevColorIn(stage, GX_CC_ZERO, GX_CC_ZERO, GX_CC_ZERO,
+                     GX_CC_C0 + tevreg_index * 2);
+    GX_SetTevAlphaIn(stage, GX_CA_ZERO, GX_CA_ZERO, GX_CA_ZERO,
+                     GX_CA_A0 + tevreg_index);
     GX_SetTevColorOp(stage, GX_TEV_ADD, GX_TB_ZERO, GX_CS_SCALE_1,
                      GX_TRUE, GX_TEVPREV);
     GX_SetTevAlphaOp(stage, GX_TEV_ADD, GX_TB_ZERO, GX_CS_SCALE_1,