[linux] Added OSMesa headless backend

platform/default/headless_backend_osmesa.cpp

@@ -0,0 +1,47 @@
+#include <mbgl/platform/default/headless_backend.hpp>
+#include <mbgl/platform/log.hpp>
+
+namespace mbgl {
+
+gl::glProc HeadlessBackend::initializeExtension(const char* name) {
+    return OSMesaGetProcAddress(name);
+}
+
+void HeadlessBackend::createContext() {
+    if (glContext == nullptr) {
+#if OSMESA_MAJOR_VERSION * 100 + OSMESA_MINOR_VERSION >= 305
+        glContext = OSMesaCreateContextExt(OSMESA_RGBA, 16, 0, 0, nullptr);
+#else
+        glContext = OSMesaCreateContext(OSMESA_RGBA, nullptr);
+#endif
+        if (glContext == nullptr) {
+            Log::Error(Event::OpenGL, "failed to create OSMesa context");
+        }
+    }
+
+    if (glContext == nullptr) {
+        throw std::runtime_error("Error creating GL context object.");
+    }
+}
+
+void HeadlessBackend::destroyContext() {
+    if (glContext) {
+        if (glContext != OSMesaGetCurrentContext()) {
+            activateContext();
+        }
+        OSMesaDestroyContext(glContext);
+        glContext = nullptr;
+    }
+}
+
+void HeadlessBackend::activateContext() {
+    if (!OSMesaMakeCurrent(glContext, &fakeBuffer, GL_UNSIGNED_BYTE, 1, 1)) {
+        throw std::runtime_error("Switching OpenGL context failed.\n");
+    }
+}
+
+void HeadlessBackend::deactivateContext() {
+    // no-op.
+}
+
+} // namespace mbgl