radeon: set swrast_renderbuffer::ColorType field when mapping renderbuffers
radeon: set swrast_renderbuffer::ColorType field when mapping renderbuffers

Fixes http://bugs.freedesktop.org/show_bug.cgi?id=47375

NOTE: This is a candidate for the 8.0 branch.

Tested-by: Barto <f7870b89ee96b44de759be51fd27a2ee0fa43315@laposte.net>

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