[multi] Avoid direct file access, read data from memory

[raysan5]

One of the big changes of raylib 3.0 was changing most of the functions reading data from files to reading data from a pre-loaded buffer, this way all external files are loaded into memory using LoadFileData()/LoadFileText() and then the buffers are processed. Consequently, multiple internal function were adapted to receive a data buffer instead of the filename.

But there are still some internal functions left:

module	function	type	details	state
text	LoadBMFont()	text	Contains external image file reference	✔️
textures	LoadDDS()	bin	DDS file format	✔️
textures	LoadPKM()	bin	PKM file format	✔️
textures	LoadKTX()	bin	KTX file format	✔️
textures	LoadPVR()	bin	PVR file format	✔️
textures	LoadASTC()	bin	ASTC file format	✔️
textures	SaveKTX()	bin	KTS file format saving	✔️
textures	ExportImageAsCode()	text	Save image data as text code file	✔️
models	LoadOBJ()	text	Parses OBJ text data but internally access .mtl file...	💬
models	LoadIQM()	bin	IQM mesh/anims file format	✔️
models	LoadModelAnimations()	bin	IQM file format anims loading...	✔️
models	ExportMesh()	text	Exports mesh data as an OBJ text file...	✔️
raudio	LoadWAV()	bin	WAV file format	✔️
raudio	SaveWAV()	bin	WAV file saving	✔️
raudio	ExportWaveAsCode()	text	Exports wave data as text code file...	✔️

Those functions have not been changed because I'm considering moving that functionality out of raylib modules to external libraries.

Note that this change does not mean adding functions for the users to allow file-formats loading from a user-provided memory buffer but support that feature internally by raylib when processing external files, just to avoid FILE operations on the module.

[raysan5]

Finally I decided not to move file-formats loading to external libraries, current raylib config.h allows disabling those files loading functionality if desired.

Probably big projects using a custom package resource fileformat prefer not using those loaders...

[raysan5]

Some functions reviewed in commit 0e56bc2

[raysan5]

LoadIQM(), LoadModelAnimations() reviewed in commit 0bcb892

[raysan5]

LoadWAV() and SaveWAV() now use drwav library, it manages file open/close internally. It's possible to use memory buffers instead but for the moment we are not using them because LoadFileData() and SaveFileData() are not available on RAUDIO_STANDALONE.

Actually, LoadOGG(), LoadFLAC() and LoadMP3() should also be reviewed to use memory buffers instead of direct file access...

[raysan5]

Redesigned ExportWaveAsCode() to use memory buffer on commit 1c5df67.

[raysan5]

LoadOBJ() had issues with material loading when .obj was not in the working directory (for example inside a resources/models directory).

Implemented solution on raylib was using chdir() to move to .obj directory before calling tinyobj_parse_obj() and return to current directory at the end of the function. It works but I don't like that solution. Also, user has no control over the file access, in case someone wants to use memory buffers for that.

There is a related issue on tinyobjloader-c project: syoyo/tinyobjloader-c#38