Large map: explored border as a continuous line along the fog edge

The game drew 2x2 px red dots per visited-sector neighbour (0x6c8580), in
64-tile steps and unrelated to the explored mask. Replaced by lines traced
from the explored cells (2x2 tiles), simplified to 0.75 px, rebuilt while
the map is open when the masks change.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/game/ui/draw.h

@@ -80,6 +80,22 @@ inline void FillCorners(void* device, float x0, float y0, float x1, float y1, ui
                                                                      quad, 4, 0);
 }
 
+// Untextured triangles, three vertices each (a triangle list), in batches the
+// device takes in one call.
+inline void FillTriangles(void* device, ScreenVertex* vertices, size_t count) {
+    constexpr DWORD kTriangleList = 4;
+    constexpr size_t kBatch = 3 * 10000;
+    void** vtable = *static_cast<void***>(device);
+    using SetTextureFn = HRESULT(__stdcall*)(void*, DWORD, void*);
+    using DrawPrimitiveFn = HRESULT(__stdcall*)(void*, DWORD, DWORD, void*, DWORD, DWORD);
+    reinterpret_cast<SetTextureFn>(vtable[kDeviceSetTexture])(device, 0, nullptr);
+    for (size_t first = 0; first < count; first += kBatch) {
+        const size_t batch = count - first < kBatch ? count - first : kBatch;
+        reinterpret_cast<DrawPrimitiveFn>(vtable[kDeviceDrawPrimitive])(
+            device, kTriangleList, kFvfScreenColourTexture, vertices + first, static_cast<DWORD>(batch), 0);
+    }
+}
+
 // Fills (x0, y0)-(x1, y1) with the ARGB colour `color`.
 inline void FillRect(void* device, float x0, float y0, float x1, float y1, uint32_t color) {
     FillCorners(device, x0, y0, x1, y1, color, color, color, color);