sdl12-compat: FIXME removal: these should be asserts, not FIXMEs.

SDL Commits
1 
From 72f4b9f7801c467ff108de48cc6731468af7ca87 Mon Sep 17 00:00:00 2001
From: "Ryan C. Gordon" <[EMAIL REDACTED]>
Date: Fri, 19 Aug 2022 17:23:43 -0400
Subject: [PATCH] FIXME removal: these should be asserts, not FIXMEs.

What alpha format would we have that isn't 2 or 4 bytes...?

Reference Issue #143.
---
 src/SDL12_compat.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/SDL12_compat.c b/src/SDL12_compat.c
index cba209ca..53a38848 100644
--- a/src/SDL12_compat.c
+++ b/src/SDL12_compat.c
@@ -6019,7 +6019,7 @@ SaveDestAlpha(SDL12_Surface *src12, SDL12_Surface *dst12, const SDL12_Rect *dstr
                 sptr = (Uint32 *) (((Uint8 *) sptr) + pitch);
             }
         } else {
-            FIXME("Unhandled dest alpha");
+            SDL_assert(!"Unhandled dest alpha");
         }
     }
 
@@ -6059,7 +6059,7 @@ RestoreDestAlpha(SDL12_Surface *dst12, Uint8 *dstalpha, const SDL12_Rect *dstrec
                 dptr = (Uint32 *) (((Uint8 *) dptr) + pitch);
             }
         } else {
-            FIXME("Unhandled dest alpha");
+            SDL_assert(!"Unhandled dest alpha");
         }
         SDL20_free(dstalpha);
     }