Rework blending equation to preserve alpha in output buffer

bda12e5fdd

src/gl/imgui_impl_opengl3.cpp

@@ -503,7 +503,7 @@ static void ImGui_ImplOpenGL3_SetupRenderState(ImDrawData* draw_data, int fb_wid
     // Setup render state: alpha-blending enabled, no face culling, no depth testing, scissor enabled, polygon fill
     glEnable(GL_BLEND);
     glBlendEquation(GL_FUNC_ADD);
-    glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
+    glBlendFuncSeparate(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA, GL_ONE, GL_ONE_MINUS_SRC_ALPHA);
     glDisable(GL_CULL_FACE);
     glDisable(GL_DEPTH_TEST);
     glDisable(GL_STENCIL_TEST);

src/vulkan.cpp

@@ -1455,8 +1455,8 @@ static void setup_swapchain_data_pipeline(struct swapchain_data *data)
    color_attachment[0].srcColorBlendFactor = VK_BLEND_FACTOR_SRC_ALPHA;
    color_attachment[0].dstColorBlendFactor = VK_BLEND_FACTOR_ONE_MINUS_SRC_ALPHA;
    color_attachment[0].colorBlendOp = VK_BLEND_OP_ADD;
-   color_attachment[0].srcAlphaBlendFactor = VK_BLEND_FACTOR_ONE_MINUS_SRC_ALPHA;
-   color_attachment[0].dstAlphaBlendFactor = VK_BLEND_FACTOR_ZERO;
+   color_attachment[0].srcAlphaBlendFactor = VK_BLEND_FACTOR_ONE;
+   color_attachment[0].dstAlphaBlendFactor = VK_BLEND_FACTOR_ONE_MINUS_SRC_ALPHA;
    color_attachment[0].alphaBlendOp = VK_BLEND_OP_ADD;
    color_attachment[0].colorWriteMask = VK_COLOR_COMPONENT_R_BIT |
       VK_COLOR_COMPONENT_G_BIT | VK_COLOR_COMPONENT_B_BIT | VK_COLOR_COMPONENT_A_BIT;