ogc: support rendering to transparent textures (#47)

Switch the EFB format to an alpha-enabled one when the texture we are
asked to render to has an alpha channel. This fixes handling of
transparency in the VVVVVV game, which makes extensive use of rendering
to textures.

src/render/ogc/SDL_render_ogc.c

@@ -43,6 +43,7 @@ typedef struct
     GXColor clear_color;
     int ops_after_present;
     bool vsync;
+    u8 efb_pixel_format;
 } OGC_RenderData;
 
 typedef struct
@@ -187,6 +188,7 @@ static void OGC_SetTextureScaleMode(SDL_Renderer *renderer,
 static int OGC_SetRenderTarget(SDL_Renderer *renderer, SDL_Texture *texture)
 {
     OGC_RenderData *data = renderer->driverdata;
+    u8 desired_efb_pixel_format = GX_PF_RGB8_Z24;
 
     if (texture) {
         if (texture->w > MAX_EFB_WIDTH || texture->h > MAX_EFB_HEIGHT) {
@@ -205,7 +207,17 @@ static int OGC_SetRenderTarget(SDL_Renderer *renderer, SDL_Texture *texture)
             SDL_LogWarn(SDL_LOG_CATEGORY_RENDER,
                         "Render target set after drawing!");
         }
+
+        if (SDL_ISPIXELFORMAT_ALPHA(texture->format)) {
+            desired_efb_pixel_format = GX_PF_RGBA6_Z24;
+        }
     }
+
+    if (desired_efb_pixel_format != data->efb_pixel_format) {
+        data->efb_pixel_format = desired_efb_pixel_format;
+        GX_SetPixelFmt(data->efb_pixel_format, GX_ZC_LINEAR);
+    }
+
     return 0;
 }
 
@@ -581,6 +593,7 @@ static SDL_Renderer *OGC_CreateRenderer(SDL_Window *window, Uint32 flags)
         return NULL;
     }
 
+    data->efb_pixel_format = GX_PF_RGB8_Z24;
     data->current_blend_mode = SDL_BLENDMODE_NONE;
     data->vsync = true;