swrast: Don't try to adjust_colors for <8bpc when handling R16, RG1616.
swrast: Don't try to adjust_colors for <8bpc when handling R16, RG1616.

The GL_RED and GL_RG were tricking this code into executing, but it's
totally unprepared for a 16-bit channel and just rescaled the values
down to 0.  We don't have anything with <8bit channels alongside >8bit
channels, so disabling it should be safe.

Reviewed-by: Brian Paul <3cb4e1df5ec4da2c7c4af7c52cec8cf340a55a10@vmware.com>

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