Fix AbstractImage::pixelSize() for depth and stencil types.

Also make the assertion more descriptive and add "to sleep better" test.
This fixes the assertion failure in FramebufferGLTest.

src/AbstractImage.cpp

@@ -97,6 +97,8 @@ std::size_t AbstractImage::pixelSize(ColorFormat format, ColorType type) {
         #ifdef MAGNUM_TARGET_GLES2
         case ColorFormat::Luminance:
         #endif
+        case ColorFormat::DepthComponent:
+        case ColorFormat::StencilIndex:
             return 1*size;
         case ColorFormat::RG:
         #ifndef MAGNUM_TARGET_GLES2
@@ -126,10 +128,8 @@ std::size_t AbstractImage::pixelSize(ColorFormat format, ColorType type) {
             return 4*size;
 
         /* Handled above */
-        case ColorFormat::DepthComponent:
-        case ColorFormat::StencilIndex:
         case ColorFormat::DepthStencil:
-            CORRADE_ASSERT_UNREACHABLE();
+            CORRADE_ASSERT(false, "AbstractImage::pixelSize(): invalid ColorType specified for depth/stencil ColorFormat", 0);
     }
 
     CORRADE_ASSERT_UNREACHABLE();

src/Test/AbstractImageTest.cpp

@@ -34,15 +34,26 @@ class AbstractImageTest: public TestSuite::Tester {
     public:
         explicit AbstractImageTest();
 
+        void pixelSize();
+
         void debugFormat();
         void debugType();
 };
 
 AbstractImageTest::AbstractImageTest() {
-    addTests({&AbstractImageTest::debugFormat,
+    addTests({&AbstractImageTest::pixelSize,
+
+              &AbstractImageTest::debugFormat,
               &AbstractImageTest::debugType});
 }
 
+void AbstractImageTest::pixelSize() {
+    CORRADE_COMPARE(AbstractImage::pixelSize(ColorFormat::RGBA, ColorType::UnsignedInt), 4*4);
+    CORRADE_COMPARE(AbstractImage::pixelSize(ColorFormat::DepthComponent, ColorType::UnsignedShort), 2);
+    CORRADE_COMPARE(AbstractImage::pixelSize(ColorFormat::StencilIndex, ColorType::UnsignedByte), 1);
+    CORRADE_COMPARE(AbstractImage::pixelSize(ColorFormat::DepthStencil, ColorType::UnsignedInt248), 4);
+}
+
 void AbstractImageTest::debugFormat() {
     std::ostringstream o;
     Debug(&o) << ColorFormat::RGBA;