lyxpreview: Simplify the color handling.

lyxpreview: Simplify the color handling.

Let's have this in trunk for testing. The real difference maker
when it comes to color is whether we use dvipng or ghostscript.

For dvipng:

  - The color info is passed as command-line arguments.

  - The tightpage option is not necessary, and since it adds
    ps specials to the output, we shouldn't use it.

For ghostscript:

  - The color info needs to be in the latex file.

  - The foreground color is set for each preview inset.

  - The background color is set by \pagecolor in the preamble,
    which is understood by pdflatex, but ignored in dvips mode.
    Thus dvips is handled with a ps special.

  - The tightpage option is necessary to crop the images.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@39797 a592a061-630c-0410-9148-cb99ea01b6c8