steamcompmgr: Handle dummy overlay plane scale for external displays

Fixes that stutter on external displays.

src/rendervulkan.cpp

@@ -2738,11 +2738,15 @@ std::shared_ptr<CVulkanTexture> vulkan_create_debug_blank_texture()
 	flags.bSampled = true;
 	flags.bTransferDst = true;
 
+	// To match Steam's scaling, which is capped at 1080p
+	int width = std::min<int>( g_nOutputWidth, 1920 );
+	int height = std::min<int>( g_nOutputHeight, 1080 );
+
 	auto texture = std::make_shared<CVulkanTexture>();
-	assert( texture->BInit( g_nOutputWidth, g_nOutputHeight, 1u, VulkanFormatToDRM( VK_FORMAT_B8G8R8A8_UNORM ), flags ) );
+	assert( texture->BInit( width, height, 1u, VulkanFormatToDRM( VK_FORMAT_B8G8R8A8_UNORM ), flags ) );
 
-	void* dst = g_device.uploadBufferData( g_nOutputWidth * g_nOutputHeight * 4 );
-	memset( dst, 0x0, g_nOutputWidth * g_nOutputHeight * 4 );
+	void* dst = g_device.uploadBufferData( width * height * 4 );
+	memset( dst, 0x0, width * height * 4 );
 
 	auto cmdBuffer = g_device.commandBuffer();
 	cmdBuffer->copyBufferToImage(g_device.uploadBuffer(), 0, 0, texture);

src/rendervulkan.hpp

@@ -717,7 +717,7 @@ class CVulkanDevice
 		return ret;
 	}
 
-	static const uint32_t upload_buffer_size = 1024 * 1024 * 4;
+	static const uint32_t upload_buffer_size = 1920 * 1080 * 4;
 
 	inline VkDevice device() { return m_device; }
 	inline VkPhysicalDevice physDev() {return m_physDev; }
@@ -738,7 +738,7 @@ class CVulkanDevice
 
 	inline void *uploadBufferData(uint32_t size)
 	{
-		assert(size < upload_buffer_size);
+		assert(size <= upload_buffer_size);
 
 		m_uploadBufferOffset = align(m_uploadBufferOffset, 16);
 		if (m_uploadBufferOffset + size > upload_buffer_size)

src/steamcompmgr.cpp

@@ -2548,8 +2548,10 @@ paint_all(bool async)
 
 		FrameInfo_t::Layer_t *layer = &frameInfo.layers[ curLayer ];
 
-		layer->scale.x = 1.0f;
-		layer->scale.y = 1.0f;
+		auto tex = vulkan_get_hacky_blank_texture();
+
+		layer->scale.x = g_nOutputWidth == tex->width() ? 1.0f : tex->width() / (float)g_nOutputWidth;
+		layer->scale.y = g_nOutputHeight == tex->height() ? 1.0f : tex->height() / (float)g_nOutputHeight;
 		layer->offset.x = 0.0f;
 		layer->offset.y = 0.0f;
 		layer->opacity = 1.0f; // BLAH
@@ -2558,8 +2560,8 @@ paint_all(bool async)
 
 		layer->colorspace = GAMESCOPE_APP_TEXTURE_COLORSPACE_LINEAR;
 		layer->ctm = nullptr;
-		layer->tex = vulkan_get_hacky_blank_texture();
-		layer->fbid = layer->tex->fbid();
+		layer->tex = tex;
+		layer->fbid = tex->fbid();
 
 		layer->filter = GamescopeUpscaleFilter::NEAREST;
 		layer->blackBorder = true;