Add IndexBuffer and update SpriteRenderer and TerrainSpriteLayer to use it

[PunkPun]

Its standard practice to use IndexBuffers in all rendering API's, not just in OpenGL. The documentation shows that the new method of rendering glDrawElements exists in OpenGL 1.1. (fyi this is considered the earliest proper OpenGL version)

Why index buffers are important? To draw a rectangle we need 2 triangles, to draw each triangle we need 3 vertices. Meaning we need 6 vertices for a square, but when we think of a square it's really just drawn from 4 points. 2 of the 6 vertices are just holding duplicated data, and do mind that a single vertex holds quite a lot of data

The solution are IndexBuffers, they allow you to reuse vertices or change their order. By using them we reduce the RAM usage on Both the GPU and CPU, as well as data they need to pass around. In TerrainSpriteLayer specific case it also significantly reduces the amount of calculations on the CPU side

I had observed the FPS timer on the RA shellmap, on bleed saw ~202, on this PR ~226 FPS
I also checked out the TS shell map bleed: ~186 PR: ~202

[PunkPun]

Added an IndexBuffer to SpriteRenderer as well

[Mailaender]

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