Fix cubemap orientation;

+Z is the front face in a cubemap, not -Z.  Currently cubemap faces are
flipped in both the X and Z directions.

Some kind of flip is required because cubemaps use a left-handed
coordinate space instead of lovr's/vulkan's right-handed coordinate
space.

Equirect does not need any changes.

(cherry picked from commit 1b1bc182bf)

etc/shaders/cubemap.frag

@@ -7,5 +7,5 @@
 layout(set = 1, binding = 1) uniform textureCube SkyboxTexture;
 
 vec4 lovrmain() {
-  return Color * getPixel(SkyboxTexture, Normal * vec3(-1, 1, 1));
+  return Color * getPixel(SkyboxTexture, Normal * vec3(1, 1, -1));
 }