cogl: Fix builds with G_DISABLE_ASSERT

Commit 25f416c13d added additional compilation warnings, including
-Werror=return-type. There are several places where this results
in build failures if `g_assert_not_reached()` is disabled at compile
time and the compiler misses a return value.

https://gitlab.gnome.org/GNOME/mutter/issues/447

cogl/cogl/cogl-bitmap-conversion.c

@@ -355,6 +355,7 @@ _cogl_bitmap_needs_short_temp_buffer (CoglPixelFormat format)
     }
 
   g_assert_not_reached ();
+  return FALSE;
 }
 
 CoglBool

cogl/cogl/driver/gl/cogl-buffer-gl.c

@@ -112,6 +112,7 @@ update_hints_to_gl_enum (CoglBuffer *buffer)
     }
 
   g_assert_not_reached ();
+  return 0;
 }
 
 static GLenum

cogl/cogl/winsys/cogl-winsys-egl.c

@@ -135,6 +135,7 @@ get_error_string (void)
      return "Invalid surface";
   default:
     g_assert_not_reached ();
+    return NULL;
   }
 }
 
@@ -219,6 +220,7 @@ _cogl_winsys_renderer_connect (CoglRenderer *renderer,
 {
   /* This function must be overridden by a platform winsys */
   g_assert_not_reached ();
+  return FALSE;
 }
 
 static void

cogl/tests/conform/test-point-size-attribute.c

@@ -30,6 +30,7 @@ calc_coord_offset (int pos, int pos_index, int point_size)
     }
 
   g_assert_not_reached ();
+  return 0;
 }
 
 static void

cogl/tests/conform/test-point-size.c

@@ -22,6 +22,7 @@ calc_coord_offset (int pos, int pos_index, int point_size)
     }
 
   g_assert_not_reached ();
+  return 0;
 }
 
 static void