The output code uses (width + n) >> k to round up to the
nearest boundary in calculating buffer sizes. Because
many people are uncertain about the relative precedence
of addition and shift operations, gcc warned about the
code as-written.

Abstract the calculation into a local variable for better
readability and add parentheses to address the warning.

It's not a large file, but it ends up being read up to three times,
and this matches style with configfile.h.

Run the code through clang_format to clean up inconsistent
whitespace. This also changed the brace style. Follow up
with some manual reformatting of long doc comments mangled
by clang_format, the config struct, and expand tabs in
the config file.

Also include global.h directly in configfile.h so the
definition of `InputParameters` is available regardless
of the include order.

Invoke the makefile so we get some build feedback on repositories
hosted under gitlab.

The code uses the MSVC-specific `_itoa` function to format
numbered filenames. Replace this with the portable `snprintf`
which is already used elsewhere, and performs bounds checks.

Also expand the allowed number of digits to avoid overflow.
The previous assumption of no more that 5 digits is exceeded
is less than an hour of video. The new maximum of 23 is enough
for a 64-bit integer, so even without configuration validation
it will work as well as the rest of the code does.

The code uses the `snprintf` function, standardized in C99.

Prior to Visual Studio 2015 this was not supported on
Microsoft Windows, so the code used a #define to map it
to the `_snprintf` extension. The Makefile had the opposite
define to undo this mapping for non-windows platforms.

Since the fallback hasn't been necessary for some years,
remove all references to _snprintf in favour of the
standard library function.

The globals.h header declared a number of global variables
as well as types, resulting in duplicate symbol warnings.
This might have compiled 15 years ago, but it doesn't
now.

Instead, move the declarations into sgi2yuv.c where they
are mostly used. The errortext global is converted to
an extern in the header so it can also be used by the
config file reader.

Move the declaration of `configinput` into the protected section
of configfile.h so it's only active when included in configfile.c.

This addresses a multily-defined symbol error with gcc 10
when configfile.h is included in other modules within the
same target executable.

This is a standard style nit and makes editing the file cleaner.

Apply a patch from derf (Timothy Terriberry) to access the image
data in row-major instead of column-major order. This vastly
improves cache coherency, speeding up the code significantly
on modern architectures.

The patch also adds a simple Makefile and cleans up various
minor issues with the code.

This is the source of the 0.4 release of Tobias Oelbaum's
sgi2yuv utility, for reading video in the format originally
used by Irix Silicon Graphics workstations.

This is from the zip file published in 2006 at
http://www.ldv.ei.tum.de/Members/tobias/videotools/sgi2yuv.zip
but excluding the compiled 32-bit Windows executable.