gl_common: make glFmt2bpp() abort on unknown formats

None of its callers actually checked the return value, and it's a given
that they call it only if it can't fail. Replace the non-sense return
value (0) by a call to abort().

libvo/gl_common.c

@@ -862,7 +862,7 @@ int glFmt2bpp(GLenum format, GLenum type)
     case GL_LUMINANCE_ALPHA:
         return 2 * component_size;
     }
-    return 0; // unknown
+    abort(); // unknown
 }
 
 /**