render/vulkan/pipeline_key: Add blending

This will become necessary when we switch away from scissoring. For the
time being, this cleans things up a bit and allows for a trivial
blending implementation for textures when that comes.

include/render/vulkan.h

@@ -150,6 +150,7 @@ enum wlr_vk_shader_source {
 struct wlr_vk_pipeline_key {
 	enum wlr_vk_shader_source source;
 	struct wlr_vk_pipeline_layout *layout;
+	enum wlr_render_blend_mode blend_mode;
 
 	// only used if source is texture
 	enum wlr_vk_texture_transform texture_transform;

render/vulkan/renderer.c

@@ -2100,6 +2100,10 @@ static bool pipeline_key_equals(const struct wlr_vk_pipeline_key *a,
 		return false;
 	}
 
+	if (a->blend_mode != b->blend_mode) {
+		return false;
+	}
+
 	if (a->source != b->source) {
 		return false;
 	}
@@ -2194,7 +2198,7 @@ struct wlr_vk_pipeline *setup_get_or_create_pipeline(
 	};
 
 	VkPipelineColorBlendAttachmentState blend_attachment = {
-		.blendEnable = true,
+		.blendEnable = key->blend_mode == WLR_RENDER_BLEND_MODE_PREMULTIPLIED,
 		// we generally work with pre-multiplied alpha
 		.srcColorBlendFactor = VK_BLEND_FACTOR_ONE,
 		.dstColorBlendFactor = VK_BLEND_FACTOR_ONE_MINUS_SRC_ALPHA,