sceneconverter: add a --map option.

Memory-maps the file and uses openMemory() instead of openFile(). For
efficient data formats (such as glTF) can avoid reading the whole blob
if only the metadata or a part of the file is needed (for example the
peak usage for --info-materials with the Buggy.glb example model went
from 8.5 MB to 991 kB, as it reads just the JSON at the start and never
even pages in the buffer blobs at the end).

This currently only works for standalone files, files that reference
external images etc. would need to have file callbacks implemented. And
it's Sunday and I'm lazy.

doc/changelog.dox

@@ -230,7 +230,9 @@ See also:
     avoid accidentally clearing default flags potentially added in the future.
 -   New @ref Trade::AbstractImporter::openMemory() function for passing
     non-temporary memory to the importer, allowing the implementations to avoid
-    allocating an internal copy
+    allocating an internal copy; for benchmarking and testing purposes it's
+    also exposed via a `--map` option in the
+    @ref magnum-sceneconverter "magnum-sceneconverter" utility
 -   Ability to convert also 1D and 3D images with the
     @ref magnum-imageconverter "magnum-imageconverter" utility, as well as
     combining layers into images of one dimension more (or vice versa) and

src/Magnum/MeshTools/sceneconverter.cpp

@@ -77,8 +77,8 @@ information.
 
 @code{.sh}
 magnum-sceneconverter [-h|--help] [-I|--importer IMPORTER]
-    [-I|--converter CONVERTER]... [--plugin-dir DIR] [--remove-duplicates]
+    [-I|--converter CONVERTER]... [--plugin-dir DIR] [--map]
-    [--remove-duplicates-fuzzy EPSILON]
+    [--remove-duplicates] [--remove-duplicates-fuzzy EPSILON]
     [-i|--importer-options key=val,key2=val2,…]
     [-c|--converter-options key=val,key2=val2,…]... [--mesh MESH]
     [--level LEVEL] [--info-animations] [--info-images] [--info-lights]
@@ -96,6 +96,8 @@ Arguments:
     @ref Trade::AnySceneImporter "AnySceneImporter")
 -   `-C`, `--converter CONVERTER` --- scene converter plugin(s)
 -   `--plugin-dir DIR` --- override base plugin dir
+-   `--map` --- memory-map the input for zero-copy import (works only for
+    standalone files)
 -   `--only-attributes "i j …"` --- include only attributes of given IDs in the
     output
 -   `--remove-duplicates` --- remove duplicate vertices using
@@ -225,6 +227,9 @@ int main(int argc, char** argv) {
         .addOption('I', "importer", "AnySceneImporter").setHelp("importer", "scene importer plugin")
         .addArrayOption('C', "converter").setHelp("converter", "scene converter plugin(s)")
         .addOption("plugin-dir").setHelp("plugin-dir", "override base plugin dir", "DIR")
+        #if defined(CORRADE_TARGET_UNIX) || (defined(CORRADE_TARGET_WINDOWS) && !defined(CORRADE_TARGET_WINDOWS_RT))
+        .addBooleanOption("map").setHelp("map", "memory-map the input for zero-copy import (works only for standalone files)")
+        #endif
         .addOption("only-attributes").setHelp("only-attributes", "include only attributes of given IDs in the output", "\"i j …\"")
         .addBooleanOption("remove-duplicates").setHelp("remove-duplicates", "remove duplicate vertices in the mesh after import")
         .addOption("remove-duplicates-fuzzy").setHelp("remove-duplicates-fuzzy", "remove duplicate vertices with fuzzy comparison in the mesh after import", "EPSILON")
@@ -292,7 +297,17 @@ used.)")
 
     std::chrono::high_resolution_clock::duration importTime;
 
-    /* Open the file */
+    /* Open the file or map it if requested */
+    #if defined(CORRADE_TARGET_UNIX) || (defined(CORRADE_TARGET_WINDOWS) && !defined(CORRADE_TARGET_WINDOWS_RT))
+    Containers::Array<const char, Utility::Directory::MapDeleter> mapped;
+    if(args.isSet("map")) {
+        Duration d{importTime};
+        if(!(mapped = Utility::Directory::mapRead(args.value("input"))) || !importer->openMemory(mapped)) {
+            Error() << "Cannot memory-map file" << args.value("input");
+            return 3;
+        }
+    } else
+    #endif
     {
         Duration d{importTime};
         if(!importer->openFile(args.value("input"))) {