Fix possible divide by zero explode edge case

Only currently apparent with vulkan but could also exist with d3d in the
right circumstances:  With vulkan mesh drawing, the axis construction lines
would disappear even when all explode params are zero.  This is almost
certainly down to normalizing a vertex offset that's coincident with
'exploderCentre', producing INFs/NaNs.  Hlsl would return explodeDir to zero
by multiplying with a zero 'exploderScale' but vulkan doesn't seem to do
that... TLDR:  Avoid dividing by zero.

Also added some lines between MeshUBOData fields to better see padding
requirements, since I wondered if I'd misaligned something.

renderdoc/data/glsl/glsl_ubos.h

@@ -35,15 +35,19 @@ BINDING(0) uniform MeshUBOData
   mat4 mvp;
   mat4 invProj;
   vec4 color;
+
   int displayFormat;
   uint homogenousInput;
   vec2 pointSpriteSize;
+
   uint rawoutput;
   uint flipY;
   float vtxExploderSNorm;
   float exploderScale;
+
   vec3 exploderCentre;
   float padding;
+
   uvec4 meshletColours[12];
 }
 INST_NAME(Mesh);

renderdoc/data/glsl/mesh.vert

@@ -43,10 +43,17 @@ void vtxExploder(in int vtxID, inout vec3 pos, inout vec3 secondary)
 {
   float nonLinearVtxExplodeScale = 4.0f * Mesh.exploderScale * Mesh.vtxExploderSNorm *
                                    Mesh.vtxExploderSNorm * Mesh.vtxExploderSNorm;
-  vec3 explodeDir = normalize(pos - Mesh.exploderCentre);
+
+  // A vertex might be coincident with our 'exploderCentre' so that, when normalized,
+  // can give us INFs/NaNs that, even if multiplied by a zero 'exploderScale', can
+  // leave us with bad numbers (as seems to be the case with glsl/vulkan, but not hlsl).
+  // Still, we should make this case safe for when we have a non-zero 'exploderScale' -
+  vec3 offset = pos - Mesh.exploderCentre;
+  float offsetDistSquared = dot(offset, offset);
+  vec3 safeExplodeDir = offset * inversesqrt(max(offsetDistSquared, FLT_EPSILON));
 
   float displacement = nonLinearVtxExplodeScale * ((float((vtxID >> 1) & 0xf) / 15.0f) * 1.5f - 0.5f);
-  pos += (explodeDir * displacement);
+  pos += (safeExplodeDir * displacement);
 
   if(Mesh.exploderScale > 0.0f)
   {

renderdoc/data/hlsl/mesh.hlsl

@@ -44,11 +44,18 @@ void vtxExploder(in uint vtxID, inout float3 pos, inout float3 secondary)
 {
   float nonLinearVtxExplodeScale =
       4.0f * exploderScale * vtxExploderSNorm * vtxExploderSNorm * vtxExploderSNorm;
-  float3 explodeDir = normalize(pos - exploderCentre);
+
+  // A vertex might be coincident with our 'exploderCentre' so that, when normalized,
+  // can give us INFs/NaNs that, even if multiplied by a zero 'exploderScale', can
+  // leave us with bad numbers (as seems to be the case with glsl/vulkan, but not hlsl).
+  // Still, we should make this case safe for when we have a non-zero 'exploderScale' -
+  float3 offset = pos - exploderCentre;
+  float offsetDistSquared = dot(offset, offset);
+  float3 safeExplodeDir = offset * rsqrt(max(offsetDistSquared, FLT_EPSILON));
 
   float displacement =
       nonLinearVtxExplodeScale * ((float((vtxID >> 1u) & 0xfu) / 15.0f) * 1.5f - 0.5f);
-  pos += (explodeDir * displacement);
+  pos += (safeExplodeDir * displacement);
 
   if(exploderScale > 0.0f)
   {