terminal,renderer: convert structs to extern

Convert Coordinate in terminal/point.zig and CellSize, ScreenSize,
GridSize, and Padding in renderer/size.zig to extern structs. All
fields are already extern-compatible types, so this gives them a
guaranteed C ABI layout with no functional change.

src/input/mouse_encode.zig

@@ -69,7 +69,7 @@ pub const Event = struct {
     pos: Pos = .{},
 
     /// Mouse position in surface-space pixels.
-    pub const Pos = struct {
+    pub const Pos = extern struct {
         x: f32 = 0,
         y: f32 = 0,
     };

src/renderer/size.zig

@@ -172,7 +172,7 @@ pub const Coordinate = union(enum) {
 ///
 /// The units for the width and height are in world space. They have to
 /// be normalized for any renderer implementation.
-pub const CellSize = struct {
+pub const CellSize = extern struct {
     width: u32,
     height: u32,
 };
@@ -180,7 +180,7 @@ pub const CellSize = struct {
 /// The dimensions of the screen that the grid is rendered to. This is the
 /// terminal screen, so it is likely a subset of the window size. The dimensions
 /// should be in pixels.
-pub const ScreenSize = struct {
+pub const ScreenSize = extern struct {
     width: u32,
     height: u32,
 
@@ -224,7 +224,7 @@ pub const ScreenSize = struct {
 };
 
 /// The dimensions of the grid itself, in rows/columns units.
-pub const GridSize = struct {
+pub const GridSize = extern struct {
     pub const Unit = terminal.size.CellCountInt;
 
     columns: Unit = 0,
@@ -257,7 +257,7 @@ pub const GridSize = struct {
 };
 
 /// The padding to add to a screen.
-pub const Padding = struct {
+pub const Padding = extern struct {
     top: u32 = 0,
     bottom: u32 = 0,
     right: u32 = 0,

src/terminal/point.zig

@@ -66,7 +66,7 @@ pub const Point = union(Tag) {
     }
 };
 
-pub const Coordinate = struct {
+pub const Coordinate = extern struct {
     /// x can use size.CellCountInt because the number of columns
     /// can't ever be more than a valid number of columns in a Page.
     x: size.CellCountInt = 0,