[rlgl] introduced rlStartBatch

[brccabral]

I was thinking about #4878 , in the solution I had to call rlSetTexture before every rlBegin, therefore I though that they should merge into one call and avoid problems in the future, so I created rlStartBatch.
I think that if I use rlBegin I would forget rlSetTexture, that's why I created rlStartBatch, so I don't forget to set both mode and textureId.
rlEndBatch only exists because sometimes there are calls to rlSetTexture(0) (maybe merge with rlEnd()?).

I tested bunnymark in RenderDoc, it looks fine (60 fps with max objects).

So, two options

1. PR [rlgl] Preserve current mode and textureId when switching mode or texture #4878 , and tell users to always call rlSetTexture/rlBegin together
2. this PR, tell users to call rlStartBatch and maybe deprecate rlSetTexture/rlBegin in the future...

[raysan5]

@brccabral I prefer to avoid that addition, raylib follows the Begin/End convention on drawing and I prefer to keep it as is.

[brccabral]

If the function name is the issue I can just rename it, the idea is more important, to not forget to set mode and textureId at the begin to avoid bug #4874 , but it is your call. Saludos