C90: correct initialisation of ARM SIMD register

[jbowler]

arm/palette_neon_intrinsics.c: change the initialisation of the "opaque
black" standard PLTE filler to be in-line rather than using a C99 style
{} initializer which calls functions.

This change also reduces the number of function calls required from 4 to
2.

Signed-off-by: John Bowler jbowler@acm.org

[jbowler]

This removes the remaining C99ism in libpng 1.6. The same issue exists in 1.8 so this can be applied there too (there should be no conflicts) but 1.8 also requires a correction to a C99ism that I accidentally introduced in the same body of code (this won't conflict with this change.)

Tested (successfully) on aarch64, Ubuntu.
Tested partially on armv7a Ubuntu; armv7a is no longer supported by Ubuntu and as a consequence my cross-build of pngvalid will not run because of changes in the glibc DLL.