Use DrawArraysInstanced instead of geometry shaders for sprite rendering

[nstlaurent]

This switches the geometry shaders used to generate vertices for sprite quads to use DrawArraysInstanced that selects the corner position in the vertex shader instead. Geometry shaders are effectively deprecated and are not supported at all on Mac OS. This uses the same basic idea where each entity sprite vertex is only generated once and is expanded and projected to a quad in the shader. This means the entity sprite vertex generation code stays practically the same with the majority of the changes being in the shader program.

This works by calling DrawArraysInstanced with a count of 4. gl_VertexID is used in the vertex shader to calculate which corner of the instance to use. Each vertex property sets the attribute divisor to one which advances the vertex once per instance of each invocation.

The performance seems to be the same but moves Helion out of the deprecated geometry shaders, simplifies the shader code by removing the geometry shader step, and allows for Mac OS support.