Make the FrameBuffer class pure abstract.

Make the FrameBuffer class pure abstract.

Sanitizer tools desire having the vtables of any class with non-pure
virtual methods, even when none of them are called in the current
linkage unit. In the case of sw::FrameBuffer, to work around this we can
make the class pure abstract by making the destructor pure virtual. Note
that the destructor still has a non-empty definition, since all non-
default destructors need a defintion, and it will get called as part of
the destructor chain.

Bug swiftshader:31

Change-Id: I7601b1a725c513ff484cd34a8965636b7f21513c
Reviewed-on: https://swiftshader-review.googlesource.com/9912
Reviewed-by: Nicolas Capens <capn@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Tested-by: Nicolas Capens <capn@google.com>