[cairo] Image surface pixel twiddling API

[T Rowley]

In implementing SVG filters in mozilla, we have the need to access and 
modify individual pixels of an image.  Our existing code relied on a 
particular byte layout, which broke when tested on a big-endian cpu for 
reasons that were obvious after taking a close read of cairo.h.

We would like an API to obtain the particular channel of a pixel.  In 
our experiments we tried two different types of API.  The first was a 
set of macros that returned a particular channel from pixel word [1], 
the second were channel offset macros that could be used to index into a 
byte pointer.  For our use, the first approach of word accessors made 
for somewhat messier source code and generated code that was slightly 
slower for x86 though faster on PPC [3].  For that reason we're more in 
favor of the channel offsets, but would like to hear feedback from 
others about what seems best.

[1] https://bugzilla.mozilla.org/attachment.cgi?id=254325

[2] https://bugzilla.mozilla.org/attachment.cgi?id=256528

[3] https://bugzilla.mozilla.org/show_bug.cgi?id=369528#c9