OGL: add check_gl_error function
OGL: add error checking to ShaderProgram
OGL: reset shader id on unload
OGL: track linking status of shader program

(GL_LINK_STATUS means something different)
OGL: either load all shader parts or none
UMVE: use util::fs api to get home dir
UMVE: fix shader load order
Merge pull request #85 from Waldteufel/umve-fix-shader-reload

Fix shader reload