swrast: initial multi-threaded span rendering

Optional parallel rendering of spans using OpenMP.
Initial implementation for aa triangles. A new option for scons is
also provided to activate the openmp support (off by default).

Signed-off-by: Brian Paul <brianp@vmware.com>
diff --git a/common.py b/common.py
index 8657030..cfee1b5 100644
--- a/common.py
+++ b/common.py
@@ -88,6 +88,7 @@
 	opts.Add('toolchain', 'compiler toolchain', default_toolchain)
 	opts.Add(BoolOption('gles', 'EXPERIMENTAL: enable OpenGL ES support', 'no'))
 	opts.Add(BoolOption('llvm', 'use LLVM', default_llvm))
+	opts.Add(BoolOption('openmp', 'EXPERIMENTAL: compile with openmp (swrast)', 'no'))
 	opts.Add(BoolOption('debug', 'DEPRECATED: debug build', 'yes'))
 	opts.Add(BoolOption('profile', 'DEPRECATED: profile build', 'no'))
 	opts.Add(BoolOption('quiet', 'DEPRECATED: profile build', 'yes'))