disable fallback font during artboard rendering

fixes #7479
The reason why the text looks different is that the editor uses a default font if it doesn't find a codepoint in the style defined font.
For example, in the font SF Pro, the symbol carriage return (\r) doesn't have a codepoint, but it does in the default font, which generates a new text run.
Share links, or our video recorder, don't have a default font so they skip that symbol.
This issue could happen with any other symbol that isn't found in the style font, so we decided to disable default fonts when rendering artboard text to guarantee they will look the same as at export time.

Diffs=
1adf3dbf4 disable fallback font during artboard rendering (#7480)

Co-authored-by: hernan <hernan@rive.app>

.rive_head

diff --git a/.rive_head b/.rive_head
index f8f1b58..b050784 100644
--- a/.rive_head
+++ b/.rive_head
@@ -1 +1 @@
-6f29a9c0c368f94a38e8077343359fdaa97663e6
+1adf3dbf4acbba7560e6e25986de126a0ca11c49

include/rive/text_engine.hpp

diff --git a/include/rive/text_engine.hpp b/include/rive/text_engine.hpp
index d244f04..a0ccd63 100644
--- a/include/rive/text_engine.hpp
+++ b/include/rive/text_engine.hpp
@@ -169,6 +169,7 @@
 
     using FallbackProc = rive::rcp<rive::Font> (*)(rive::Span<const rive::Unichar>);
     static FallbackProc gFallbackProc;
+    static bool gFallbackProcEnabled;
 
 protected:
     Font(const LineMetrics& lm) : m_lineMetrics(lm) {}

src/text/font_hb.cpp

diff --git a/src/text/font_hb.cpp b/src/text/font_hb.cpp
index 2bb77e6..8bd2324 100644
--- a/src/text/font_hb.cpp
+++ b/src/text/font_hb.cpp
@@ -22,6 +22,8 @@
 // Initialized to null. Client can set this to a callback.
 rive::Font::FallbackProc rive::Font::gFallbackProc;
 
+bool rive::Font::gFallbackProcEnabled = true;
+
 rive::rcp<rive::Font> HBFont::Decode(rive::Span<const uint8_t> span)
 {
     auto blob = hb_blob_create_or_fail((const char*)span.data(),
@@ -554,7 +556,7 @@
 
             auto end = gr.glyphs.end();
             auto iter = std::find(gr.glyphs.begin(), end, 0);
-            if (!gFallbackProc || iter == end)
+            if (!gFallbackProc || iter == end || !gFallbackProcEnabled)
             {
                 if (gr.glyphs.size() > 0)
                 {