Add dashed circle and line functions.

[ecraven]

Add the following two functions, each taking the number of total segments and the number of on/off segments for dashes.

E.g. on=1,off=3 would result in 1 segment being drawn, then 3 not being drawn, then 1 drawn, ...

[ocornut]

Maybe the on/off thing should be expressed in a different way. User may want control of the segment length? Maybe 1 float for segment length, 1 float for "ratio" (1.0f contiguous line, 0.5f half dashes).

Those primitives are too heavy as is, especially with anti-aliasing. Both in term of CPU and GPU cost (mostly CPU). If the atlas textures contained pre-baked variations of dashed lines with various distance, those functions could use the closest one and scale it and use textures maybe? It would only be a little noticeable on the tip of each dash but would dramatically reduce CPU and vertex cost.