Load an unlimited amount of loose music (#1171)

* Load an unlimited amount of loose music

Binary blobs can now store 128 tracks, and music can now be read from
loose files. Unfortunately, the two cannot be used together -- loose
music can still only use 16 tracks. This commit attempts to fix that,
allowing for music playback by filename.

* Rework music track loading

The current "loose songs" system does not load files from custom
assets. This is from the #412 PR, and is NOT the "extra loose songs"
system that this PR implements, but they both work in the same area so
I decided to squash both of these bugs at once (plus, it helped clean
up the code and make a saner system, so...)

This commit also allows "loose music" to override music stored in the
`vvvvvvmusic.vvv` binary blob. When reading a blob, it will first check
if a loose file exists at the current path it's trying to find a track
at; if it exists, it will load that instead. If the blob does not
exist, the system will try to locate all expected built-in tracks
directly from the assets folder instead of doing that during blob
loading.

The `play` command has some safety removed due to an old exploit which
needs to be kept -- `music(5b)` should play track 5. This is used in
many levels and cannot be broken; unfortunately this means that we no
longer get the nice error when a song fails to be played, but that's
better than breaking a large amount of levels.

This commit does NOT address loading a "extra" loose track overtop of
a custom blob track, because with the current tooling, and the way it
will most likely continue to work in the future, every single custom
track in blobs have a completely empty path. If someone decides to
modify their blob in a hex editor or makes their own tooling, this
issue may show up again, but that is a problem which can and probably
should be addressed at a later date. The required code to solve such a
problem would be far more than what this PR should address, and for a
situation so unlikely, it's easy enough to just avoid adding the same
custom song twice, one in a music file, and one loose, because if
you're making a level, you're probably going to use one or the other.

* Fix mentioned issues from review

desktop_version/src/FileSystemUtils.h

@@ -5,6 +5,7 @@
 class binaryBlob;
 
 #include <stddef.h>
+#include <SDL_rwops.h>
 
 // Forward declaration, including the entirety of tinyxml2.h across all files this file is included in is unnecessary
 namespace tinyxml2 { class XMLDocument; }
@@ -35,6 +36,8 @@ bool FILESYSTEM_areAssetsInSameRealDir(const char* filenameA, const char* filena
 bool FILESYSTEM_saveFile(const char* name, const unsigned char* data, size_t len);
 void FILESYSTEM_loadFileToMemory(const char *name, unsigned char **mem,
                                  size_t *len);
+
+SDL_RWops* FILESYSTEM_loadAssetRWops(const char* name);
 void FILESYSTEM_loadAssetToMemory(
     const char* name,
     unsigned char** mem,