From the extension spec:

"8) What value should gl_MaxDrawBuffers in the shading language report?"

"RESOLVE: It should match MAX_DRAW_BUFFERS_EXT from the API. None of the API or
GLSL specifications explicitly state the linkage between API and SL constants,
but it seems logical that one would expect them to match, regardless of whether
or not an extension directive is used in the shading language."

TRAC #23509

Signed-off-by: Shannon Woods
Signed-off-by: Nicolas Capens
Authored-by: Jamie Madill