On desktop, blit depth when not cleared

Workaround for #1283, makes Jeanne d'Arc playable afaict.
 
If hrydgard/native#185 is not merged (and native updated), it will actually make Jeanne d'Arc (and any other games not clearing depth when switching) *significantly* worse on cards with OpenGL < 4.3.

This could work on GLES3 but it probably has a perf penalty even for games that don't need it.

-[Unknown]