check for return-value of makeCurrent()

Closes: CID#75581
don't allocate unused memory

Closes CID#75582, CID#75585
initialize VertexBuffer::type

Closes CID#75583, CID#75587, CID#75589
Initialize model::m_size_change_flag

Closes CID#27971
initialize GEMglFogfv:pname

use GL_FOG_START, as this seems to be a no-op
mark unterminated "default" as intended

Closes CID#75591