Added max color buffer count to framebuffer info

If the extension WEBGL_draw_buffers is available the fragment shader can
output multiple colors. The maximum count of color buffers is
implementation dependent.
The maximum count of color buffers is 1 if the extension is not available.

It is displayed as part of the framebuffer info.
Make pipeline taller to accomodate new entry.
Merge branch 'max-color-buffers'