use suffixed literals instead of casts to construct 'GLfloat' values

use suffixed literals instead of casts to construct 'GLfloat' values

The `GLfloat` type is 32-bit IEEE-754 floating-point.¹ As of C99, the `float`
type is also 32-bit IEEE-754 floating-point on all platforms with support.² In
contemporary usage, this is effectively all C99 platforms that support
floating-point arithmetic. Graphviz has been compiled in C99 mode since commit
fe3f9411d2c59b463ab1b64eecfd19f2db55d2fc. Putting those things together, we can
abbreviate the way we write `GLfloat` literals, leading to more readable code.

¹ https://www.khronos.org/opengl/wiki/OpenGL_Type
² https://en.wikipedia.org/wiki/C99#IEEE_754_floating-point_support