In particular this fixes Blitter::Options state having uninitialized
bits after the bitfield, and Blitter::State having uninitialized padding
bytes after Options so 'sourceFormat' is 32-bit aligned.

Defines a Memset<T> class to be used as the first base class of cache
key types, which makes it explicit that their underlying memory will be
fully initialized before any member constructors are run.

Also adds is_memcmparable<T> for checking if memcmp() can be used to
implement operator==() for cache keys. It's equivalent to
std::is_trivially_copyable except it provides a fallback for STL
implementations that don't support it.

Bug: b/134932616
Change-Id: I6569e78b380e67aee02d5bfd39d80d210bd225fd
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/32929
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>