fix(render): Fix grayscale volumes (#8)

Fixes grayscale volumes not being rendered correctly 
(https://www.shadertoy.com/view/st2XD1)

Fixes #3

Author: sshcrack

shadertoy/OpenGL.cpp

@@ -576,8 +576,9 @@ class GLVolumeObject final : public TextureObject {
         assert(data);
         glTexParameteri(GL_TEXTURE_3D, GL_TEXTURE_BASE_LEVEL, 0);
         glTexParameteri(GL_TEXTURE_3D, GL_TEXTURE_MAX_LEVEL, static_cast<GLint>(std::log2(size)));
-        GLenum format = channels == 1 ? GL_R8 : GL_RGBA;
-        glTexImage3D(GL_TEXTURE_3D, 0, format, static_cast<GLsizei>(size), static_cast<GLsizei>(size), static_cast<GLsizei>(size),
+        GLenum internalFormat = channels == 1 ? GL_R8 : GL_RGBA;
+        GLenum format = channels == 1 ? GL_RED : GL_RGBA;
+        glTexImage3D(GL_TEXTURE_3D, 0, internalFormat, static_cast<GLsizei>(size), static_cast<GLsizei>(size), static_cast<GLsizei>(size),
                      0, format, GL_UNSIGNED_BYTE, data);  // R8G8B8A8
         glGenerateMipmap(GL_TEXTURE_3D);
         glBindTexture(GL_TEXTURE_3D, GL_NONE);