fixup: Change GL_Alpha_Threshold use to value on state tracker

No need to use the global here. Fixes build without OpenGL.

Author: laanwj

code/graphics/vulkan/VulkanDraw.cpp

@@ -1501,8 +1501,6 @@ void VulkanDrawManager::drawSphere(material* material_def)
 } // namespace vulkan
 } // namespace graphics
 
-// GL_alpha_threshold is defined in gropengl.cpp
-extern float GL_alpha_threshold;
 
 // PostProcessing_override is defined in globalincs/systemvars.cpp
 extern bool PostProcessing_override;
@@ -1556,7 +1554,7 @@ void vulkan_set_default_material_uniforms(material* material_info)
 	}
 
 	// Alpha threshold
-	data->alphaThreshold = GL_alpha_threshold;
+	data->alphaThreshold = getStateTracker()->getAlphaThreshold();
 
 	// Color from material
 	vec4 clr = material_info->get_color();

code/graphics/vulkan/VulkanState.cpp

@@ -290,9 +290,6 @@ void VulkanStateTracker::applyDynamicState()
 } // namespace vulkan
 } // namespace graphics
 
-// GL_alpha_threshold is defined in gropengl.cpp
-extern float GL_alpha_threshold;
-
 namespace graphics {
 namespace vulkan {
 
@@ -319,9 +316,9 @@ void vulkan_zbias(int bias)
 int vulkan_alpha_mask_set(int mode, float alpha)
 {
 	if (mode) {
-		GL_alpha_threshold = alpha;
+		getStateTracker()->setAlphaThreshold(alpha);
 	} else {
-		GL_alpha_threshold = 0.0f;
+		getStateTracker()->setAlphaThreshold(0.0f);
 	}
 	return mode;
 }

code/graphics/vulkan/VulkanState.h

@@ -176,6 +176,9 @@ class VulkanStateTracker {
 	void setCullMode(bool enabled) { m_cullEnabled = enabled; }
 	bool getCullMode() const { return m_cullEnabled; }
 
+	void setAlphaThreshold(float threshold) { m_alphaThreshold = threshold; }
+	float getAlphaThreshold() const { return m_alphaThreshold; }
+
 	/**
 	 * @brief Set color attachment count for current render pass
 	 */
@@ -237,6 +240,7 @@ class VulkanStateTracker {
 	gr_zbuffer_type m_zbufferMode = ZBUFFER_TYPE_NONE;
 	int m_stencilMode = 0;
 	bool m_cullEnabled = true;
+	float m_alphaThreshold = 0.0f;
 	uint32_t m_colorAttachmentCount = 1;
 	vk::SampleCountFlagBits m_currentSampleCount = vk::SampleCountFlagBits::e1;