DebugTools: fallback to Texture::subImage() on desktop.

I still have to fill up the missing parts to make it orthogonal, but NO
DAMN TIME.

src/Magnum/DebugTools/TextureImage.cpp

@@ -26,6 +26,8 @@
 #include "TextureImage.h"
 
 #include "Magnum/BufferImage.h"
+#include "Magnum/Context.h"
+#include "Magnum/Extensions.h"
 #include "Magnum/Framebuffer.h"
 #include "Magnum/Image.h"
 #include "Magnum/Texture.h"
@@ -92,6 +94,13 @@ FloatReinterpretShader::FloatReinterpretShader() {
 #endif
 
 void textureSubImage(Texture2D& texture, const Int level, const Range2Di& range, Image2D& image) {
+    #ifndef MAGNUM_TARGET_GLES
+    if(Context::current().isExtensionSupported<Extensions::GL::ARB::get_texture_sub_image>()) {
+        texture.subImage(level, range, image);
+        return;
+    }
+    #endif
+
     #ifdef MAGNUM_TARGET_GLES
     if(image.type() == PixelType::Float) {
         const PixelFormat imageFormat = image.format();
@@ -165,6 +174,13 @@ void textureSubImage(CubeMapTexture& texture, const CubeMapCoordinate coordinate
 
 #ifndef MAGNUM_TARGET_GLES2
 void textureSubImage(Texture2D& texture, const Int level, const Range2Di& range, BufferImage2D& image, const BufferUsage usage) {
+    #ifndef MAGNUM_TARGET_GLES
+    if(Context::current().isExtensionSupported<Extensions::GL::ARB::get_texture_sub_image>()) {
+        texture.subImage(level, range, image, usage);
+        return;
+    }
+    #endif
+
     Framebuffer fb{range};
     fb.attachTexture(Framebuffer::ColorAttachment{0}, texture, level)
       .read(range, image, usage);

src/Magnum/DebugTools/TextureImage.h

@@ -39,7 +39,8 @@ namespace Magnum { namespace DebugTools {
 
 Emulates @ref Texture::subImage() "*Texture::subImage()" call on platforms that
 don't support it (such as OpenGL ES) by creating a framebuffer object and using
-@ref Framebuffer::read().
+@ref Framebuffer::read(). On desktop OpenGL, if @extension{ARB,get_texture_sub_image}
+is available, it's just an alias to @ref Texture::subImage() "*Texture::subImage()".
 
 Note that only @ref Magnum::PixelFormat "PixelFormat" and @ref PixelType values
 that are marked as framebuffer readable are supported. In addition, on OpenGL