mesa: _mesa_Begin/EndConditionalRender() functions

For GL_NV_conditional_render and GL3.

The drawing functions don't check the query object yet.
No API dispatch yet.
diff --git a/src/mesa/SConscript b/src/mesa/SConscript
index f4e0b98..7035bdc 100644
--- a/src/mesa/SConscript
+++ b/src/mesa/SConscript
@@ -38,6 +38,7 @@
 		'main/clear.c',
 		'main/clip.c',
 		'main/colortab.c',
+		'main/condrender.c',
 		'main/context.c',
 		'main/convolve.c',
 		'main/cpuinfo.c',