fixup: use our own

Author: laanwj

code/graphics/vulkan/VulkanDraw.cpp

@@ -1501,8 +1501,7 @@ void VulkanDrawManager::drawSphere(material* material_def)
 } // namespace vulkan
 } // namespace graphics
 
-// GL_alpha_threshold is defined in gropengl.cpp
-extern float GL_alpha_threshold;
+extern float VK_alpha_threshold;
 
 // PostProcessing_override is defined in globalincs/systemvars.cpp
 extern bool PostProcessing_override;
@@ -1556,7 +1555,7 @@ void vulkan_set_default_material_uniforms(material* material_info)
 	}
 
 	// Alpha threshold
-	data->alphaThreshold = GL_alpha_threshold;
+	data->alphaThreshold = VK_alpha_threshold;
 
 	// Color from material
 	vec4 clr = material_info->get_color();

code/graphics/vulkan/VulkanState.cpp

@@ -290,8 +290,7 @@ void VulkanStateTracker::applyDynamicState()
 } // namespace vulkan
 } // namespace graphics
 
-// GL_alpha_threshold is defined in gropengl.cpp
-extern float GL_alpha_threshold;
+float VK_alpha_threshold = 0.0f;
 
 namespace graphics {
 namespace vulkan {
@@ -319,9 +318,9 @@ void vulkan_zbias(int bias)
 int vulkan_alpha_mask_set(int mode, float alpha)
 {
 	if (mode) {
-		GL_alpha_threshold = alpha;
+		VK_alpha_threshold = alpha;
 	} else {
-		GL_alpha_threshold = 0.0f;
+		VK_alpha_threshold = 0.0f;
 	}
 	return mode;
 }