VS: Add VS_SHADER_OUTPUT_HEADER_FILE and VS_SHADER_VARIABLE_NAME properties.

The VS_SHADER_OUTPUT_HEADER_FILE property is the name of the generated
header file containing the object code of the shader.

The VS_SHADER_VARIABLE_NAME property is the name of the variable
containing the object code in the above header file.

Signed-off-by: Kevin M. Godby <kevin@godby.org>