DebugTools: handle and test size 0 in bufferData()/bufferSubData() for all cases

src/Magnum/DebugTools/BufferData.h

@@ -60,13 +60,14 @@ WebGL 2.0 it's just an alias to @ref GL::Buffer::subData().
     WebGL 1.0.
 */
 template<class T> Containers::Array<T> inline bufferSubData(GL::Buffer& buffer, GLintptr offset, GLsizeiptr size) {
+    if(!size) return {};
     #if !defined(MAGNUM_TARGET_GLES) || defined(MAGNUM_TARGET_WEBGL)
     Containers::Array<char> data = buffer.subData(offset, size*sizeof(T));
     CORRADE_INTERNAL_ASSERT(!data.deleter());
     return Containers::Array<T>{reinterpret_cast<T*>(data.release()), std::size_t(size)};
     #else
     Containers::Array<T> data{std::size_t(size)};
-    if(size) Implementation::bufferSubData(buffer, offset, size*sizeof(T), data);
+    Implementation::bufferSubData(buffer, offset, size*sizeof(T), data);
     return data;
     #endif
 }

src/Magnum/DebugTools/Test/BufferDataGLTest.cpp

@@ -53,6 +53,11 @@ void BufferDataGLTest::data() {
     #endif
 
     GL::Buffer buffer;
+
+    const Containers::Array<Int> emptyContents = bufferData<Int>(buffer);
+    MAGNUM_VERIFY_NO_GL_ERROR();
+    CORRADE_VERIFY(emptyContents.isEmpty());
+
     buffer.setData(Data, GL::BufferUsage::StaticDraw);
     const Containers::Array<Int> contents = bufferData<Int>(buffer);
     MAGNUM_VERIFY_NO_GL_ERROR();
@@ -72,6 +77,10 @@ void BufferDataGLTest::subData() {
     MAGNUM_VERIFY_NO_GL_ERROR();
     CORRADE_COMPARE_AS(contents, Containers::arrayView(Data).slice(1, 4),
         TestSuite::Compare::Container);
+
+    const Containers::Array<Int> emptyContents = bufferSubData<Int>(buffer, 4, 0);
+    MAGNUM_VERIFY_NO_GL_ERROR();
+    CORRADE_VERIFY(emptyContents.isEmpty());
 }
 
 }}}}