GLSL codegen: Improve printing of floats
GLSL codegen: Improve printing of floats

- use fewer digits after the decimal point for integral numbers
- use more digits to fractional number to ensure correct round trip
  conversions (binary->decimal->binary)
- add a few tests for this


Former-commit-id: 8780737f250ef6c3a266620b17190dd2d62ce4e1
darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide