[macOS] Fix searching for SKIA_OPENGL_LIBRARY
[macOS] Fix searching for SKIA_OPENGL_LIBRARY

In macOS, if the user's filesystem is **case-sensitive**, then laf CAN'T locate
`/System/Library/Frameworks/OpenGL.framework/`, which results in link failures.
(aseprite/aseprite#1776)

This is because `cmake/FindSkia.cmake` tries to locate `/System/Library/Frameworks/opengl.framework/`.
(Notice the lowercase "opengl" instead of "OpenGL").

This commit fixes such error on macOS with a **case-sensitive**
filesystem. Tested on Macbook Pro Retina 2014 with Mojave + APFS
(case-sensitive).

aseprite/laf,aseprite/laf