Text: allow calling GlyphCache::reserve() for cache with default glyph 0.

The default glyph 0 is not positioned anywhere, so it won't interfere
with atlas creation.

src/Text/GlyphCache.cpp

@@ -69,7 +69,8 @@ void GlyphCache::initialize(const TextureFormat internalFormat, const Vector2i&
 }
 
 std::vector<Rectanglei> GlyphCache::reserve(const std::vector<Vector2i>& sizes) {
-    CORRADE_ASSERT(glyphs.empty(), "Text::GlyphCache::reserve(): reserving space in non-empty cache is not yet implemented", {});
+    CORRADE_ASSERT((glyphs.size() == 1 && glyphs.at(0) == std::pair<Vector2i, Rectanglei>()),
+        "Text::GlyphCache::reserve(): reserving space in non-empty cache is not yet implemented", {});
     glyphs.reserve(glyphs.size() + sizes.size());
     return TextureTools::atlas(_size, sizes, _padding);
 }

src/Text/Test/GlyphCacheGLTest.cpp

@@ -33,11 +33,13 @@ class GlyphCacheGLTest: public Magnum::Test::AbstractOpenGLTester {
 
         void initialize();
         void access();
+        void reserve();
 };
 
 GlyphCacheGLTest::GlyphCacheGLTest() {
     addTests({&GlyphCacheGLTest::initialize,
-              &GlyphCacheGLTest::access});
+              &GlyphCacheGLTest::access,
+              &GlyphCacheGLTest::reserve});
 }
 
 void GlyphCacheGLTest::initialize() {
@@ -78,6 +80,13 @@ void GlyphCacheGLTest::access() {
     CORRADE_COMPARE(rectangle, Rectanglei({10, 10}, {23, 45}));
 }
 
+void GlyphCacheGLTest::reserve() {
+    Text::GlyphCache cache(Vector2i(236));
+
+    /* Verify that this works for "empty" cache */
+    CORRADE_VERIFY(!cache.reserve({{5, 3}}).empty());
+}
+
 }}}
 
 CORRADE_TEST_MAIN(Magnum::Text::Test::GlyphCacheGLTest)