splash: New buffer accessors for SplashBitmap

Add alphaAt and dataAt public methods for simple and type corrected
access into underlying alpha and data buffers.

Adapt SplashBitmap.cc with these new functions and static_cast as
demonstration of use to handle values that do not fit in int.
7 jobs for bitmap_ptr_at in 5 minutes and 9 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build
passed #4299617
build

00:05:09

passed #4299622
build_android

00:00:58

passed #4299618
build_clang_libcpp

00:05:00

passed #4299621
build_clazy

00:03:33

passed #4299620
build_mingw64_fedora

00:02:11

passed #4299619
build_ubuntu_16_04

00:02:19

passed #4299616
clang_format

00:01:11