COMMON: Move definition of math constants to scummsys header.

This should have no effect on common/math.h as scummsys is included at
the top anyway, but this will fix compilation on some toolchains when
strict ANSI language compliance is enabled i.e. -std=c++11, rather than
-std=gnu11.

This causes an issue as a side effect is to set a guard which disables
various non-standard defines in the system headers i.e. the M_PI and
M_SQRT1_2 constant definitions in the MinGW x86_64 toolchain on
buildbot.

By moving this into scummsys.h after the point of system math.h
inclusion, we can ensure that M_PI etc. are always present.
COMMON: Add comment explaining reason for defining math constants.
Merge pull request #363 from digitall/EnsurePI

Fix compilation with C++11 enabled on Win64 x86_64 toolchain... and probably others.
GRAPHICS: Silence conversion warnings by using an explicit cast.
GRAPHICS: Slight formatting fixes in thumbnail_intern.cpp.
GRAPHICS: Allow to query a Surface describing a subarea in Surface.
COMMON: Define all Unix98/BSD math constants in scummsys.h.

This should fix compilation on mingw64 with --enable-c++11 and also make sure
the constants are present on other systems for the uncommon case they might be
missing.
SCUMM: Prefer getBasePtr over direct Surface::pixels access.
AGOS: Prefer getBasePtr over direct Surface::pixels access.
CGE: Prefer getBasePtr over direct Surface::pixels access.
COMPOSER: Prefer getBasePtr over direct Surface::pixels access.
DRASCULA: Prefer getBasePtr over direct Surface::pixels access.
GOB: Prefer getBasePtr over direct Surface::pixels access.
HOPKINS: Prefer getBasePtr over direct Surface::pixels access.
HUGO: Prefer getBasePtr over direct Surface::pixels access.
LASTEXPRESS: Prefer getBasePtr over direct Surface::pixels access.
MADE: Prefer getBasePtr over direct Surface::pixels access.
MOHAWK: Prefer getBasePtr over direct Surface::pixels access.
NEVERHOOD: Prefer getBasePtr over direct Surface::pixels access.
MORTEVIELLE: Prefer getBasePtr over direct Surface::pixels access.