[sddm-theme] Add slot to configure a logo to be shown on the SDDM login screen

Summary: The Breeze SDDM theme now shows a logo that can be configured by distro vendors or curious users or anyone else.

Test Plan:
Test configuration and defaults.

Default logo:
{F7028488}

Example with a distro logo:
{F7023244}

Reviewers: #vdg, #plasma, filipf, ngraham

Reviewed By: #vdg, filipf, ngraham

Subscribers: GB_2, Codezela, filipf, davidedmundson, broulik, ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D22458

sddm-theme/Main.qml

@@ -411,6 +411,46 @@ PlasmaCore.ColorScope {
             }
         }
 
+        Image {
+            id: logo
+            visible: config.showlogo == "shown"
+            source: config.logo
+            anchors.horizontalCenter: parent.horizontalCenter
+            anchors.bottom: footer.top
+            anchors.bottomMargin: units.largeSpacing
+            asynchronous: true
+            sourceSize.height: height
+            opacity: loginScreenRoot.uiVisible ? 0 : 1
+            fillMode: Image.PreserveAspectFit
+            height: Math.round(units.gridUnit * 3.5)
+            Behavior on opacity {
+                OpacityAnimator {
+                    duration: units.longDuration
+                    easing.type: Easing.InOutQuad
+                }
+            }
+        }
+
+        DropShadow {
+            id: logoShadow
+            anchors.fill: logo
+            source: logo 
+            visible: !softwareRendering && config.showlogo == "shown"
+            horizontalOffset: 1
+            verticalOffset: 1
+            radius: 6
+            samples: 14
+            spread: 0.3
+            color: "black" // matches Breeze window decoration and desktopcontainment
+            opacity: loginScreenRoot.uiVisible ? 0 : 1
+            Behavior on opacity {
+                OpacityAnimator {
+                    duration: units.longDuration
+                    easing.type: Easing.InOutQuad
+                }
+            }
+        }
+
         //Footer
         RowLayout {
             id: footer

sddm-theme/default-logo.svg

@@ -0,0 +1,4 @@
+<svg width="4626" height="1044" viewBox="0 0 4626 1044" fill="none" xmlns="http://www.w3.org/2000/svg">
+<path d="M712.182 149.566C686.396 149.566 665.636 170.325 665.636 196.111C665.636 221.897 686.396 242.656 712.182 242.656C737.968 242.656 758.727 221.897 758.727 196.111C758.727 170.325 737.968 149.566 712.182 149.566ZM1038 149.566L944.909 242.656L1084.55 382.293L944.909 521.929L1038 615.02L1177.64 475.384L1270.73 382.293L1038 149.566ZM595.818 428.838C557.139 428.838 526 459.977 526 498.656C526 537.336 557.139 568.475 595.818 568.475C634.497 568.475 665.636 537.336 665.636 498.656C665.636 459.977 634.497 428.838 595.818 428.838V428.838ZM805.273 708.111C753.7 708.111 712.182 749.63 712.182 801.202C712.182 852.774 753.7 894.293 805.273 894.293C856.845 894.293 898.364 852.774 898.364 801.202C898.364 749.63 856.845 708.111 805.273 708.111Z" fill="white"/>
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+</svg>

sddm-theme/theme.conf.cmake

@@ -1,4 +1,6 @@
 [General]
+showlogo=hidden
+logo=${KDE_INSTALL_FULL_DATADIR}/sddm/themes/breeze/default-logo.svg
 type=image
 color=#1d99f3
 fontSize=10