mesa: reset the source packing when creating temp transfer image

Commit 4b249d2ee (mesa: Handle transferOps in texstore_rgba) introduced
proper transferops handling, but in updating the source to the newly
allocated temporary image neglected to reset the source packing. Set it
to the default which should be appropriate for the floats used.

Fixes: 4b249d2ee (mesa: Handle transferOps in texstore_rgba)
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=91173
Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Cc: "10.5 10.6" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Chris Forbes <chrisf@ijw.co.nz>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>

src/mesa/main/texstore.c

@@ -787,6 +787,7 @@ texstore_rgba(TEXSTORE_PARAMS)
       srcType = GL_FLOAT;
       srcRowStride = srcWidth * 4 * sizeof(float);
       srcMesaFormat = RGBA32_FLOAT;
+      srcPacking = &ctx->DefaultPacking;
    }
 
    src = (GLubyte *)