Vulkan/D3D12: Use BLEND_OP_MAX for alpha channel blending

Change the alpha blend operation from ADD to MAX in the blend-enabled
pipeline states for both Vulkan and D3D12. This ensures the render
target alpha channel is always 1.0 (since the background establishes
alpha=1.0 and max(src, 1.0) preserves it) while leaving RGB blending
with SRC_ALPHA / ONE_MINUS_SRC_ALPHA completely unchanged.

Author: MajorPainTheCactus

gfx/drivers/d3d12.c

@@ -456,7 +456,7 @@ static D3D12_RENDER_TARGET_BLEND_DESC d3d12_blend_enable_desc = {
    D3D12_BLEND_OP_ADD,
    D3D12_BLEND_SRC_ALPHA,
    D3D12_BLEND_INV_SRC_ALPHA,
-   D3D12_BLEND_OP_ADD,
+   D3D12_BLEND_OP_MAX,
    D3D12_LOGIC_OP_NOOP,
    D3D12_COLOR_WRITE_ENABLE_ALL,
 };

gfx/drivers/vulkan.c

@@ -2628,7 +2628,7 @@ static void vulkan_init_pipelines(vk_t *vk)
    blend_attachment.colorBlendOp        = VK_BLEND_OP_ADD;
    blend_attachment.srcAlphaBlendFactor = VK_BLEND_FACTOR_SRC_ALPHA;
    blend_attachment.dstAlphaBlendFactor = VK_BLEND_FACTOR_ONE_MINUS_SRC_ALPHA;
-   blend_attachment.alphaBlendOp        = VK_BLEND_OP_ADD;
+   blend_attachment.alphaBlendOp        = VK_BLEND_OP_MAX;
 
    /* Glyph pipeline */
    module_info.codeSize                 = sizeof(font_frag);