shaders: Add GLSL port of accum_mask.

Author: CrossVR

res/gamedata/shaders/gl/accum_mask.vs

@@ -0,0 +1,10 @@
+#include "common.h"
+
+out gl_PerVertex { vec4 gl_Position; };
+
+layout(location = POSITION) in float4 P;
+
+void main ()
+{
+	gl_Position = mul ( m_WVP, P );
+}