[core] Enable dynamic updates of index buffers.

src/mbgl/gl/context.cpp

@@ -226,16 +226,25 @@ void Context::updateVertexBuffer(UniqueBuffer& buffer, const void* data, std::si
     MBGL_CHECK_ERROR(glBufferSubData(GL_ARRAY_BUFFER, 0, size, data));
 }
 
-UniqueBuffer Context::createIndexBuffer(const void* data, std::size_t size) {
+UniqueBuffer Context::createIndexBuffer(const void* data, std::size_t size, const BufferUsage usage) {
     BufferID id = 0;
     MBGL_CHECK_ERROR(glGenBuffers(1, &id));
     UniqueBuffer result { std::move(id), { this } };
     bindVertexArray = 0;
     globalVertexArrayState.indexBuffer = result;
-    MBGL_CHECK_ERROR(glBufferData(GL_ELEMENT_ARRAY_BUFFER, size, data, GL_STATIC_DRAW));
+    MBGL_CHECK_ERROR(glBufferData(GL_ELEMENT_ARRAY_BUFFER, size, data, static_cast<GLenum>(usage)));
     return result;
 }
 
+void Context::updateIndexBuffer(UniqueBuffer& buffer, const void* data, std::size_t size) {
+    // Be sure to unbind any existing vertex array object before binding the index buffer
+    // so that we don't mess up another VAO
+    bindVertexArray = 0;
+    globalVertexArrayState.indexBuffer = buffer;
+    MBGL_CHECK_ERROR(glBufferSubData(GL_ELEMENT_ARRAY_BUFFER, 0, size, data));
+}
+
+
 UniqueTexture Context::createTexture() {
     if (pooledTextures.empty()) {
         pooledTextures.resize(TextureMax);

src/mbgl/gl/context.hpp

@@ -75,11 +75,18 @@ public:
     }
 
     template <class DrawMode>
-    IndexBuffer<DrawMode> createIndexBuffer(IndexVector<DrawMode>&& v) {
+    IndexBuffer<DrawMode> createIndexBuffer(IndexVector<DrawMode>&& v, const BufferUsage usage=BufferUsage::StaticDraw) {
         return IndexBuffer<DrawMode> {
-            createIndexBuffer(v.data(), v.byteSize())
+            v.indexSize(),
+            createIndexBuffer(v.data(), v.byteSize(), usage)
         };
     }
+    
+    template <class DrawMode>
+    void updateIndexBuffer(IndexBuffer<DrawMode>& buffer, IndexVector<DrawMode>&& v) {
+        assert(v.indexSize() == buffer.indexCount);
+        updateIndexBuffer(buffer.buffer, v.data(), v.byteSize());
+    }
 
     template <RenderbufferType type>
     Renderbuffer<type> createRenderbuffer(const Size size) {
@@ -250,7 +257,8 @@ private:
 
     UniqueBuffer createVertexBuffer(const void* data, std::size_t size, const BufferUsage usage);
     void updateVertexBuffer(UniqueBuffer& buffer, const void* data, std::size_t size);
-    UniqueBuffer createIndexBuffer(const void* data, std::size_t size);
+    UniqueBuffer createIndexBuffer(const void* data, std::size_t size, const BufferUsage usage);
+    void updateIndexBuffer(UniqueBuffer& buffer, const void* data, std::size_t size);
     UniqueTexture createTexture(Size size, const void* data, TextureFormat, TextureUnit);
     void updateTexture(TextureID, Size size, const void* data, TextureFormat, TextureUnit);
     UniqueFramebuffer createFramebuffer();

src/mbgl/gl/index_buffer.hpp

@@ -35,6 +35,7 @@ private:
 template <class DrawMode>
 class IndexBuffer {
 public:
+    std::size_t indexCount;
     UniqueBuffer buffer;
 };