Add quirks for libmgba-dev

Some of these are workarounds for bugs in the package which should be fixed, but for now this MR should serve to unblock the analysis.

• patches/libmgba-dev.patch: Fix mismatched CXX_GUARD_START in .../renderers/video-software.h. Will report upstream.
• mgba-util/png-io.h does #undefine restrict, so it cannot be processed in the same run as any headers that use restrict. Worked around using a virtual package to process png-io.h separately from the other headers.
• Added libpng-dev and defined USE_PNG for png-io.h.
• Added mgba-util/common.h to preamble as it should be included before any other headers.
• Defined HAVE_LOCALE to avoid a conflicting redefinition of locale_t.
• Skipped foreign platform headers under platform/*/*.h. These are included conditionally by platform detection macros in (for example) mgba-util/threading.h and are not meant to be included directly. Will consider fixing libmgba-dev to exclude those headers that are unusable on Debian, but they should be harmless.