Reducing draw calls in textoverlay example

[GDBobby]

if this pull request is denied, I'd still recommend changing line 107 to

VkDeviceSize bufferSize = TEXTOVERLAY_MAX_CHAR_COUNT * sizeof(glm::vec4) * 4;

or renaming TEXTOVERLAY_MAX_CHAR_COUNT to TEXTOVERLAY_MAX_VERTEX_COUNT

addText should have protections for vertexBuffer overflow (situations in which numLetters >= TEXTOVERLAY_MAX_CHAR_COUNT), but I'm assuming that's out of scope for this example.

there's still room for other easy optimizations.
like an index buffer,
or a separate buffer containing font data as a whole, reducing uploaded data and cpu calculations.

[SaschaWillems]

Thanks for your PR. Your contribution is very much appreciated.

As for further improvements: I want to keep this sample as simple as possible, it's kinda like a precursor to the ImGui sample. So I'm fine with the way it is (after this PR).