Update uint64_t documentation

[timblakely]

Looks like the image_layer_rendering.md doc needs to be updated to reflect the change in uint64_t from

struct uint64_t {
  vec4 low, high;
};

to

struct uint64_t {
  highp uvec2 value;
};

Other uintX_t values/definitions on that page might need updating as well (haven't tried them yet).