[linux] Added OSMesa headless backend

include/mbgl/gl/implementation.hpp

@@ -9,6 +9,8 @@
     #else
         #define MBGL_USE_CGL 1
     #endif
+#elif defined(__OSMESA__)
+    #define MBGL_USE_OSMESA 1
 #else
     #define MBGL_USE_GLX 1
 #endif

include/mbgl/platform/default/headless_backend.hpp

@@ -12,6 +12,8 @@ typedef struct __GLXcontextRec* GLXContext;
 typedef struct __GLXFBConfigRec* GLXFBConfig;
 typedef long unsigned int XID;
 typedef XID GLXPbuffer;
+#elif MBGL_USE_OSMESA
+#include <GL/osmesa.h>
 #endif
 
 #include <mbgl/map/backend.hpp>
@@ -71,6 +73,11 @@ private:
     GLXPbuffer glxPbuffer = 0;
 #endif
 
+#if MBGL_USE_OSMESA
+    OSMesaContext glContext = nullptr;
+    GLubyte fakeBuffer = 0;
+#endif
+
     std::function<void(MapChange)> mapChangeCallback;
 
 };