Change multisampling count to 0, improved fix/workaround for AMD GPUs (#26)

Part of fixing graphical rendering bugs: https://github.com/iceiix/steven/issues/25
Corrupted graphics on AMD video cards: https://github.com/Thinkofname/steven/issues/74

The previous workaround set NUM_SAMPLES to 2, but this added an extra texel fetch. Setting to 1 reproduces the 16px blue striped unexpected behavior. Setting to 0 samples avoids the extra fetch and the striping.

This is allowed per http://docs.gl/gl3/glTexImage2DMultisample:
> samples specifies the number of samples in the image and must be in the range zero to GL_MAX_SAMPLES - 1.

src/render/mod.rs

@@ -43,7 +43,7 @@ use std::sync::mpsc;
 const ATLAS_SIZE: usize = 1024;
 
 // TEMP
-const NUM_SAMPLES: i32 = 2;
+const NUM_SAMPLES: i32 = 0;
 
 pub struct Camera {
     pub pos: cgmath::Point3<f64>,

src/render/shaders/trans_frag.glsl

@@ -15,6 +15,7 @@ void main() {
     for (int i = 1; i < samples; i++) {
         col += texelFetch(tcolor, C, i);
     }
+    if (samples != 0)
     col /= float(samples);
 
     float r = accum.a;