This commit performs two main changes:
* Alters the shaders to use only strictly conformant GLSL 2.0.
* Alters the Windows UI to use ANGLE via GL ES 2.0 and EGL 1.4.
This commit also drops official support for Windows XP, since ANGLE
requires a non-XP toolset to build. It is still possible to build
SolveSpace for Windows XP using:
cmake -T v120_xp -DOPENGL=1
This commit makes common external packages always be included through
find_package to eliminate differences in variables set, wraps
find_package for vendored libraries on Windows to factor out common
code, and removes miscellaneous useless code elsewhere in dependency
handling.
This also fixes a problem where pkg-config would pick up `build`
libraries instead of `host` when cross-compiling.
whitequark