mesa: new format_pack.c code

This code packs colors, Z, stencil, etc. in the various mesa pixel
formats.  Will be used for things like glDrawPixels, glTexImage,
glAccum, etc.
diff --git a/src/mesa/SConscript b/src/mesa/SConscript
index bcb6a24..5c50189 100644
--- a/src/mesa/SConscript
+++ b/src/mesa/SConscript
@@ -74,6 +74,7 @@
     'main/ffvertex_prog.c',
     'main/fog.c',
     'main/formats.c',
+    'main/format_pack.c',
     'main/format_unpack.c',
     'main/framebuffer.c',
     'main/get.c',